enhance(#1577): WebFetch/WebSearch injection isolation + opt-in blocking (#1585)

* fix(#1577): isolate WebFetch/WebSearch ingress + opt-in injection blocking

Split A of #1573 (security-critical). Scans WebFetch/WebSearch output (the
largest untrusted channel) in gsd-read-injection-scanner; shared
untrusted-input-boundary reference @-included by the 8 ingest agents
(randomized per-wrap delimiters, in-prompt self-scan guard, task-anchoring);
opt-in security.injection_blocking (default advisory — non-breaking).

arXiv: 2506.05739 (PPA), 2507.15219 (PromptArmor), 2504.20472 (Referencing), 2503.00061 (defense-in-depth).

* fix(#1577): address review — honest blocking docs, config key, ADR, property test, revert localized

- A1: rewrote the opt-in-blocking doc + Security changeset honestly — the PostToolUse hook is a
  circuit-breaker (halts the agent's next step), NOT a redactor; it does not scrub content already
  in the transcript. The prompt-level data/instruction boundary is the primary control.
- A2: registered security.injection_blocking in the config schema + defaults manifests (default
  false) + an e2e config-roundtrip test; the dotted setter writes the nested shape the hook reads.
- A3: reverted the 4 hand-edited localized security-model.md (canonical EN only, per convention).
- A5: ADR-1577 (untrusted-input boundary + opt-in blocking; redaction-vs-circuit-breaker rationale).
- A6: property test — scanner never crashes / only emits valid JSON on unicode/large/malformed input.
- Also: inventory (untrusted-input-boundary.md) + agent-size baseline (8 ingest agents) +
  drift-guard matcher update (Read -> Read|WebFetch|WebSearch). A7 (content<20 early-exit) left as
  the noted pre-existing follow-up.

* fix(#1577): allowlist untrusted-input-boundary.md in injection-scan CI gate

The new reference quotes injection phrases ('ignore previous instructions',
'you are now…') as examples agents must NOT comply with, tripping the repo's
own prompt-injection-scan.sh diff gate (the standalone 'security' CI job, red
on HEAD). Allowlist it alongside the other security docs (security-model.md,
TEST-EXAMPLES.md) that legitimately demonstrate injection patterns. The JS
scanner test doesn't scan references/, so only the shell gate needed it.

Verified: scan --diff origin/next -> 0 findings; scanner JS test 15/15.

* fix(#1577): cover AC #2's gsd-ui-researcher + gsd-assumptions-analyzer

trek-e Major 1: the @-included set dropped two AC #2 agents. Restore them so
no named web-ingress agent is uncovered, keeping the two justified additions
(gsd-ai-researcher, gsd-domain-researcher). Final set = AC's 8 + 2 = 10.
 - gsd-ui-researcher carries the full WebSearch/WebFetch + MCP-fetch toolset.
 - gsd-assumptions-analyzer reads 5-15 codebase source files (external/source-
   document ingress per the boundary), though it has no web tools.
INGEST_AGENTS in the isolation test now asserts all 10; size baselines
regenerated (+60 bytes each, both well under the DEFAULT cap); changeset
reworded 8 -> 10.

Verified: untrusted-input-isolation 14/14; agent-size-budget 39/39.

* docs(#1577): document security.injection_blocking + boundary seam

trek-e Major 2 + Minor:
 - docs/CONFIGURATION.md: add the top-level security.injection_blocking key to
   the Full Schema and a Security Settings subsection, distinguishing it from
   the workflow.security_* namespace; honest circuit-breaker-not-redactor
   framing matching ADR-1577 / security-model.
 - CONTEXT.md: add the 'Untrusted-input boundary' seam glossary entry.

Verified: lint:docs ok; config-field-docs + contributor-standards green.

* test(#1577): make read-injection property test git-text, not binary

trek-e nit (and more): the file embedded a raw U+FFFF AND a raw NUL byte as
degenerate-edge inputs. The NUL is what actually made git classify it binary
(git binary = NUL in first 8K). Replace both with text-safe escapes that keep
the identical runtime values: '\\x00' and String.fromCodePoint(0xFFFF). File
now diffs/blames line-by-line.

Verified: property test 2/2; no NUL/raw-noncharacter bytes remain.

* docs(#1577): align untrusted boundary docs

Name all 10 ingress agents in INVENTORY/security-model and allowlist the intentional read-injection property corpus for the prompt-injection scanner.

* docs(#1577): align ADR ingest agent count

Update ADR-1577 from 8 to 10 ingest agents so it matches the actual boundary include set and the rest of the docs.

---------

Co-authored-by: Tom Boucher <trekkie@nomorestars.com>

docs/explanation/security-model.md

@@ -153,10 +153,35 @@ false-positive block on a legitimate planning write would be more disruptive
 than a missed injection in a secondary scan layer.
 
 **Runtime hook: `gsd-read-injection-scanner.js`.** This hook fires on the
-output of every Read tool call. It scans the *content that was just read* for
-injected instructions in untrusted content — catching cases where an attacker
-has embedded instructions in a file that GSD is about to incorporate into an
-agent's context.
+output of every Read, WebFetch, and WebSearch tool call. It scans the *content
+that was just read or fetched* for injected instructions in untrusted content —
+catching cases where an attacker has embedded instructions in a file or remote
+resource that GSD is about to incorporate into an agent's context. The 10
+research and doc-ingest agents additionally carry a shared `<security_context>`
+data/instruction boundary (defined in
+`gsd-core/references/untrusted-input-boundary.md`): `gsd-project-researcher`,
+`gsd-phase-researcher`, `gsd-ui-researcher`, `gsd-assumptions-analyzer`,
+`gsd-advisor-researcher`, `gsd-doc-classifier`, `gsd-doc-synthesizer`,
+`gsd-research-synthesizer`, `gsd-ai-researcher`, and `gsd-domain-researcher`.
+Any content fetched or read by those agents is treated as data, never as
+instructions, regardless of what the content claims to be.
+
+**Opt-in blocking (`security.injection_blocking`).** By default all injection
+detections are advisory-only (logged, not blocked). Setting
+`security.injection_blocking = true` in `.planning/config.json` (a registered
+config key — `gsd config-set security.injection_blocking true`) upgrades
+HIGH-confidence detections to **blocking**. Be precise about what this does: the
+scanner is a **PostToolUse** hook, so it runs *after* the Read/WebFetch/WebSearch
+has already executed and the fetched content is already in the model's transcript.
+Blocking does **not** retroactively redact that content — it emits
+`decision: "block"`, which halts the agent's next step and feeds the detection back
+as the reason, so the agent is stopped from acting further on the flagged result
+instead of silently continuing. LOW detections remain advisory under this setting.
+This flag is opt-in; the default (advisory-only) is preserved to avoid breaking
+existing workflows. The prompt-level boundary above (treat fetched text as data,
+never instructions) is the layer that keeps an injection from being *followed* even
+while it sits in context; the hook is a coarse pattern pre-filter and circuit-breaker,
+not a redactor.
 
 **CI scanner.** `prompt-injection-scan.security.test.cjs` scans all agent, workflow,
 and command files for embedded injection vectors as part of the test suite.
@@ -167,11 +192,14 @@ instruction.
 ### Read Injection Scanner vs Prompt Guard
 
 The two hooks cover complementary surfaces. `gsd-prompt-guard.js` watches
-*writes to planning artifacts* — it catches injection being planted. 
-`gsd-read-injection-scanner.js` watches *reads of any file* — it catches
+*writes to planning artifacts* — it catches injection being planted.
+`gsd-read-injection-scanner.js` watches *reads and remote fetches* — it catches
 injection being ingested from external content (a dependency's README, a
-third-party config file, a user-provided document). Together they bracket
-the ingest → store → re-read lifecycle.
+third-party config file, a user-provided document, or any URL fetched via
+WebFetch or WebSearch). The in-prompt `<security_context>` boundary in research
+agents provides an additional containment layer: even if an injected string
+reaches an agent, it is structurally separated from the instruction region.
+Together these controls bracket the ingest → store → re-read lifecycle.
 
 ---
 
@@ -228,10 +256,14 @@ not hard-stopping on a detection.
 
 **What the prompt injection defences do not eliminate:** A sufficiently
 creative injection that does not match known patterns, or an injection that
-arrives through a channel the hooks do not cover (for example, content injected
-into a dependency's published README that is read by a subagent browsing
-documentation). Defence in depth means each layer makes the attack harder,
-not that any single layer makes it impossible.
+arrives through a channel the hooks do not cover. The previously uncovered
+channel of content injected into a dependency's published README and read by a
+subagent browsing documentation is now scanned at ingress by
+`gsd-read-injection-scanner.js` (which covers WebFetch and WebSearch output)
+and structurally isolated in-prompt by the `<security_context>` boundary in
+research agents — but novel jailbreaks and low-signal injections may still pass
+undetected. Defence in depth means each layer makes the attack harder, not that
+any single layer makes it impossible.
 
 **Reporting vulnerabilities.** Report via private GitHub security advisory at
 `https://github.com/open-gsd/gsd-core/security/advisories/new`. Do not open