fix(#3099): emit LAST_ACTIVITY_UNPARSEABLE diagnostic for unusable last_activity (#3139)

* fix(#3099): emit LAST_ACTIVITY_UNPARSEABLE diagnostic when last_activity is present but unparseable

parseActivityTimestamp returned null for both absent AND present-but-unusable
last_activity values, silently suppressing the idle-stranded recommendation.
Per ADR-1411's amendment (corrupt is not absent), the fallback stays but a
diagnostic is now emitted via the warnUnusableInput seam.

- Added LAST_ACTIVITY_UNPARSEABLE to UNUSABLE_REASON enum + prose
- Wired warnUnusableInput into detectSignals when lastActivityRaw is truthy
  but parseActivityTimestamp returned null
- Updated UNUSABLE_REASON lock test
- Added 5 regression tests (unusable→diagnostic, absent→silent, well-formed→silent, dedup)

* chore(#3099): add changeset fragment

* chore(#3099): backfill changeset PR number 3139

---------

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

src/smart-entry.cts

@@ -51,6 +51,9 @@ const { stateExtractField } = stateDocument;
 // eslint-disable-next-line @typescript-eslint/no-require-imports
 import phaseId = require('./phase-id.cjs');
 const { comparePhaseNum, extractPhaseToken, normalizePhaseName, phaseTokenMatches } = phaseId;
+// eslint-disable-next-line @typescript-eslint/no-require-imports
+import unusableInput = require('./unusable-input.cjs');
+const { warnUnusableInput, UNUSABLE_REASON } = unusableInput;
 
 // ─── Types ────────────────────────────────────────────────────────────────────
 
@@ -355,7 +358,16 @@ export function detectSignals(cwd: string, now: () => number = Date.now): SmartE
   // Stale = no recorded activity for IDLE_STALE_MS. Used only by idle-stranded.
   // Computed here (with the clock seam) so the pure classify() stays a function
   // of (signals, staleActivity) and detectSignals owns all disk reads.
+  // #3099 (ADR-1411 amendment): if last_activity is present but unparseable,
+  // emit a diagnostic so the silent fallback (stale_activity: false) is visible.
+  // The fallback itself stays — continuity is correct, the silence was the defect.
   const lastActivityMs = parseActivityTimestamp(lastActivityRaw);
+  if (lastActivityRaw && lastActivityMs === null) {
+    warnUnusableInput({
+      reason: UNUSABLE_REASON.LAST_ACTIVITY_UNPARSEABLE,
+      source: paths.state,
+    });
+  }
   const staleActivity = lastActivityMs !== null && now() - lastActivityMs > IDLE_STALE_MS;
 
   // Verify-failed may be signalled either by STATE.md status or by a failed