enhance(#2573): stamp STATE.md with its commit and surface a freshness hint (#2622)

* enhance(#2573): stamp STATE.md with its commit and surface a commit-age freshness hint

Adds a `state_head` stamp to STATE.md and derives a tri-state commit-age
freshness proxy (state_commits_behind / state_commit_stale) through
state.cjs's readStateHeadFreshness, surfaced on smart-entry signals and as
health W024. The proxy is advisory: classify() deliberately does NOT consume
it (ADR-1787 locks the classification/routing boundary — a signal, not a route).

Composes with #3099 and #1882 (both merged to next after this branch): the
commit-age proxy reads `state_head` while the LAST_ACTIVITY_UNPARSEABLE
diagnostic reads `last_activity` — two different fields, not "two staleness
signals on one field." A new regression test asserts a STATE.md carrying both
an unparseable last_activity AND a valid state_head resolves each independently
(diagnostic fires once; freshness reads state_head, commits_behind 0).

Rebased onto next (flattened): resolved the add/add conflicts in
src/smart-entry.cts (kept both the #2573 freshness import/derivation and the
#3099 diagnostic import/call) and tests/smart-entry.unit.test.cjs (kept both
describe blocks). Drift-ack for health.md's W024 row is unchanged (12348 B).
Tests: smart-entry 62, state/state-transition/health/verify 639, all pass.

* chore(#2573): allowlist health-validation test in the prompt-injection scan

The scanner's `exec('` code-execution pattern matches the benign
`re.exec('<phase-id>')` RegExp method calls in the phase-ID grammar tests
(pre-existing: 16 such calls on next, this PR adds none). The file entered the
diff-mode scan's changed-file set only because #2573's W024 state_head
assertions touch it. Allowlist it alongside the other test files that carry
pattern-matching content as data (same DEFECT.PROMPT-INJECTION-SCAN-COLLISION
class). Scanner self-test 38/0; diff scan 14 files, 0 findings.

gsd-core/workflows/health.md

@@ -186,6 +186,7 @@ Report final status.
 | W009 | warning | Phase has Validation Architecture in RESEARCH.md but no VALIDATION.md | No |
 | W018 | warning | MILESTONES.md missing entry for archived milestone snapshot | Yes (`--backfill`) |
 | W019 | warning | Unrecognized .planning/ root file — not a canonical GSD artifact | No |
+| W024 | warning | STATE.md was written many commits ago — treat its contents as approximate | No |
 | I001 | info | Plan without SUMMARY (may be in progress) | No |
 
 </error_codes>