chore(#3546): migrate hook advisory assertions onto typed output surfaces (#4167)

* chore(#3546): migrate hook advisory assertions onto typed output surfaces

Add additive typed fields to 5 hook scripts' PreToolUse/PostToolUse
advisory output alongside the existing additionalContext prose:

- gsd-read-guard.js: code ('READ_BEFORE_EDIT'), fileName
- gsd-context-monitor.js: severity ('warning'|'critical')
- gsd-prompt-guard.js: findings ([{ruleId, match}], module-local RULE_IDS
  + renderFinding mapper mirroring gsd-read-injection-scanner.js's #3523
  pattern)
- gsd-read-injection-scanner.js: severity ('LOW'|'HIGH'), source (its
  findings array already existed from #3523)
- gsd-workflow-guard.js: code ('WORKFLOW_ADVISORY') on the advisory leg,
  distinct from the existing force-add block leg's code

additionalContext stays byte-identical in every hook (verified per-hook
against the pristine HEAD version across a spread of payload shapes).

Migrates all 20 assertion sites named in the issue off
additionalContext.includes(...)/assert.match(...) substring-matching
onto the new typed fields, per CONTRIBUTING.md's prohibition on raw
text matching on test outputs.

Closes #3546

* test: fix undersized commit-class timeout in gsd-statusline.test.cjs's commitN helper

Surfaced by gsd-test on the #3546 checkpoint: `commitN()`'s loop called
gitOrThrow(['add','-A']/['commit',...]) without a timeoutMs override, so
each call used DEFAULT_GIT_TIMEOUT_MS (15s) -- a bound git-fixture.cjs's
own doc comment says is sized for plumbing reads (rev-parse/branch/log),
not write-heavy add/commit spawns. That file already documents the exact
same defect class from a prior incident (PR #3323) and exports
GIT_FIXTURE_TIMEOUT_MS (60s) for fixture-construction call sites -
commitN just wasn't using it. Observed failure: `git commit -m filler 9`
timed out under normal bench load, unrelated to any of this PR's own
diff (hooks/*.js + 5 other test files).

Not a flake: root-caused to the timeout bound being sized for the wrong
call class, per this repo's no-flakes rule.

* chore(#3546): backfill changeset PR number (#4167)

---------

Co-authored-by: sim <sim@local>

.changeset/mellow-mice-jump.md

@@ -0,0 +1,5 @@
+---
+type: Added
+pr: 4167
+---
+**Prompt-injection guard advisory output now carries a typed `findings` array** — `gsd-prompt-guard.js`'s `.planning/` write-scan advisory now emits `findings: [{ruleId, match}]` records (mirroring the pattern `gsd-read-injection-scanner.js` already ships) alongside its existing `additionalContext` prose, rendered through a single mapper so the two can never drift. (#3546)