fix(#3696): report the last_activity invariant and make the verdict gateable with --strict (#3844)

* test(#3696): failing-first coverage for the last_activity invariant and --strict exit status

* fix(#3696): report the last_activity invariant and make the verdict gateable with --strict

* fix(#3696): agree with the real reader on last_activity, and stop reporting structure as truncation

* chore(#3696): backfill changeset PR number

---------

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

docs/FEATURES.md

@@ -1796,7 +1796,7 @@ Test suite that scans all agent, workflow, and command files for embedded inject
 
 ### 69. STATE.md Consistency Gates
 
-**Commands:** `state validate`, `state sync [--verify]`, `state planned-phase --phase N --plans N`
+**Commands:** `state validate [--strict]`, `state sync [--verify]`, `state planned-phase --phase N --plans N`
 
 **Purpose:** Detect and repair drift between STATE.md and the actual filesystem, preventing cascading errors from stale state.
 
@@ -1805,6 +1805,8 @@ Test suite that scans all agent, workflow, and command files for embedded inject
 - REQ-STATE-02: `state sync` MUST reconstruct STATE.md from actual project state on disk
 - REQ-STATE-03: `state sync --verify` MUST perform a dry-run showing proposed changes without writing
 - REQ-STATE-04: `state planned-phase` MUST record the state transition after plan-phase completes (Planned/Ready to execute)
+- REQ-STATE-05: `state validate` MUST report a `Last activity` value that no reader can parse, rather than validating clean
+- REQ-STATE-06: `state validate --strict` MUST reflect `valid` in the process exit status, leaving the default exit status unchanged
 
 **Produces:**
 | Artifact | Description |