feat(#3970): per-task external-tracker content-resolution seam (#4000)

* feat(#3970): per-task external-tracker content-resolution seam

Implements ADR-3646 (Phase 1, #3970): a `<task tracker-id="...">` attribute
plus a new optional `taskContentResolver` capability-manifest field let a
capability resolve a task's action/verify/acceptance-criteria/read_first/done
content from an external issue tracker instead of PLAN.md's inline body.

- src/plan-document.cts: parses the `tracker-id` attribute into `PlanTask.trackerId`
- src/task-content-resolution.cts: new leaf module — split/find/build/resolve,
  with a hard-halt (throw) contract on ambiguous/failed/timeout/malformed
  resolution, never a silent fallback to possibly-stale inline text
- src/task-command-router.cts: new `task resolve-content --plan --task-id --raw`
  CLI verb wiring the module into a real process exit code
- gsd-core/bin/lib/capability-validator.cjs: validates the new
  `taskContentResolver` manifest field (feature-role only, cross-capability
  trackerPrefix uniqueness)
- gsd-core/workflows/execute-plan.md, gsd-core/references/loop-hook-dispatch.md,
  docs/reference/capability-manifest.md: wire the seam into the per-task loop
  and document it as a new `execute:task` point outside the existing
  contribution/step/gate vocabulary (unconditional in autonomous mode)

Closes #3970

* fix(#3970): gate checkpoint tasks out of content resolution, close trackerPrefix grammar parity gap, cover path-traversal guard

Standards/Spec code-review pass on the task-content-resolution seam (ADR-3646
Phase 1) found three defects:

1. execute-plan.md's task-content-resolution bullet fired on any
   tracker-id-bearing task with no check that it wasn't type="checkpoint:*",
   contradicting ADR-3646 Decision 1 (a checkpoint task must never enter
   resolve-content). plan-document.cts already parses trackerId: null
   unconditionally for checkpoint tasks; only the workflow prose needed the
   fix, so the bullet now explicitly excludes checkpoint tasks.

2. task-content-resolution.cts's parseResolverDeclaration accepted any
   non-empty trackerPrefix with no grammar check, while capability-
   validator.cjs's KEBAB_RE enforces kebab-case at install time — a
   Generative Fix Divergence gap. Added the same grammar (as a literal
   regex, documented as intentionally not shared across the .cts/.cjs build
   boundary) plus a parity test asserting the two surfaces agree across a
   valid/invalid trackerPrefix table.

3. task-command-router.cts's routeResolveContent path-traversal guard on
   --plan had zero test coverage. Added a test exercising a
   ../../../etc/passwit-shaped path and asserting the USAGE rejection names
   the offending path.

* fix(#3970): sanitize resolver diagnostics and cap resolver timeoutMs

Two findings caught by an isolated security-review pass on the task
content resolution seam:

- ResolverFailedError/ResolverMalformedOutputError embedded raw,
  unsanitized subprocess stderr/stdout (attacker/model-influenced via
  the tracker-id argv token) into .message. A hostile or buggy resolver
  could smuggle a newline plus a forged "Error: " line, or terminal
  escape sequences, into a diagnostic io.cjs's error() writes verbatim
  to stderr. Fixed at the constructor (task-content-resolution.cts) via
  io.cjs's existing formatDiagnosticToken(), so every caller of
  resolveTaskContent gets a safe .message by construction.

- capability-validator.cjs's validateTaskContentResolverFields had no
  upper bound on taskContentResolver.invoke.timeoutMs, letting a
  manifest declare an effectively unbounded value and defeat the
  "bounded subprocess" design intent. Added a 120000ms ceiling specific
  to this field, without touching the shared isPositiveIntegerMs()
  helper (still used unbounded by the reviewer lane's timeoutFloorMs
  and probe timeoutMs).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* fix(#3970): fix gsd-test failures — stale prose allowlist line and stderr-vs-message assertion

gsd-test (remote dockerized matrix) came back red with 5 failures on this
PR; all five are real defects, fixed here.

- tests/no-bare-gsd-tools-command-position.test.cjs: PROSE_ALLOWLIST's
  execute-plan.md entry pointed at line 415, which ffc190df4's
  checkpoint-exclusion caveat (added near line 221) shifted down by one
  line. The actual "validated downstream by gsd-tools uat
  classify-coverage" descriptive mention now sits at line 416. Updated
  the allowlist entry's line number to match.

- tests/task-command-router-resolve-content.test.cjs: the path-traversal
  test asserted the outside-project-scope diagnostic against the thrown
  ExitError's own .message. io.cts's error() (ADR-3889) writes its
  human-readable message to fd 2 via writeAllSync and then throws a bare
  `new ExitError(1)` with no message argument — by design, so the
  exception carries no duplicate text and the thrown ExitError's message
  defaults to "process exit 1" (cli-exit.cts's ExitError constructor).
  Root cause was the test, not the source: task-command-router.cjs's
  outside-project-scope rejection already calls error() correctly and the
  diagnostic text is genuinely emitted, just on fd 2, not on the
  exception. Fixed the test to capture fd-2 writes (mirroring
  tests/estimate-calibrate.test.cjs's runCalibrateExpectError and this
  same file's own captureStdout for fd 1) and assert against the captured
  stderr text instead of err.message. This was masked locally because a
  manual `node -e` sanity check that only inspects the caught exception's
  .message cannot see what the real node:test run actually failed on.

Emitted-Drift-Ack-Growth: execute-plan.md — adds the ADR-3646 task-content-resolution bullet and checkpoint-exclusion caveat to the per-task execute loop; a real behavioral prose addition, not incidental bloat.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* docs(#3970): backfill changeset PR number (pr:0 -> pr:4000)

---------

Co-authored-by: sim <sim@local>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

gsd-core/references/loop-hook-dispatch.md

@@ -96,3 +96,25 @@ Honor `onError` if the check itself errors: `skip` means treat as non-blocking a
 
 If `activeHooks` is absent, null, or an empty array, skip silently and continue to the next
 step in the workflow. No output to the user is needed.
+
+## The `execute:task` point (a different shape)
+
+`execute:task` exists below wave granularity — it is evaluated once per task, inside the
+`execute:wave:pre` / `execute:wave:post` bracket, immediately before that task's `read_first`
+gate. It is **not** one of the 12 points documented above, does not appear in `steps` /
+`contributions` / `gates`, and is never dispatched through `gsd_run loop render-hooks <point>` or
+this file's `activeHooks` envelope.
+
+Instead, a capability declares task-content resolution directly in its manifest body via
+`taskContentResolver` (`trackerPrefix` + a bounded `invoke`) — see
+[Capability manifest reference](../../docs/reference/capability-manifest.md). `execute-plan.md`'s
+per-task loop calls `gsd_run task resolve-content --plan <path> --task-id <tracker-id> --raw`
+directly, an unconditional, required subprocess invocation with a real, binding exit code —
+never a prose-dispatched `step`/`gate` entry chosen from an `activeHooks` array.
+
+This point always runs — there is no `when` config gate and no autonomous-mode elision. That is
+deliberate, not an oversight: the twelve points above are best-effort prose dispatch, which
+`execute:task`'s hard-halt safety property cannot be built on top of (a missed dispatch is
+indistinguishable from a legitimate resolver-empty fallback). See
+[ADR-3646](../../docs/adr/3646-per-task-content-resolution-seam.md) for the full rationale,
+including why a `kind: "gate"` shape was rejected outright.