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>

tests/smart-entry.unit.test.cjs

@@ -549,3 +549,89 @@ describe('#2427 — roadmap-grounded completion + tightened status regex', () =>
       `legacy fallback must still reject completion when current_phase < total_phases. Got: ${result.situation}`);
   });
 });
+
+// ---------------------------------------------------------------------------
+// #3099: unusable last_activity emits a diagnostic (ADR-1411 amendment:
+// corrupt is not absent — the fallback stays, the silence is the defect)
+// ---------------------------------------------------------------------------
+
+describe('#3099: unusable last_activity emits a diagnostic', () => {
+  const {
+    _resetUnusableInputWarningsForTests,
+    _unusableInputEmissionCountForTests,
+  } = require('../gsd-core/bin/lib/unusable-input.cjs');
+
+  function makeStateWithActivity(activity) {
+    return [
+      '---',
+      'status: executing',
+      `last_activity: ${activity}`,
+      '---',
+      '',
+      '# Project State',
+      '',
+      'Phase: 1',
+      '',
+    ].join('\n');
+  }
+
+  test('unusable last_activity still resolves stale_activity: false (fallback unchanged)', () => {
+    _resetUnusableInputWarningsForTests();
+    const dir = track(makeProject({
+      state: makeStateWithActivity('yesterday - did some work'),
+    }));
+    const signals = detectSignals(dir);
+    assert.equal(signals.stale_activity, false,
+      'unusable last_activity must still resolve stale_activity: false (continuity is correct)');
+  });
+
+  test('unusable last_activity emits LAST_ACTIVITY_UNPARSEABLE diagnostic', () => {
+    _resetUnusableInputWarningsForTests();
+    const dir = track(makeProject({
+      state: makeStateWithActivity('not-a-date-at-all'),
+    }));
+    detectSignals(dir);
+    assert.equal(_unusableInputEmissionCountForTests(), 1,
+      'unusable last_activity must emit exactly one diagnostic');
+  });
+
+  test('absent last_activity emits nothing (distinguishable from unusable)', () => {
+    _resetUnusableInputWarningsForTests();
+    const dir = track(makeProject({
+      state: [
+        '---',
+        'status: executing',
+        '---',
+        '',
+        '# Project State',
+        '',
+        'Phase: 1',
+        '',
+      ].join('\n'),
+    }));
+    detectSignals(dir);
+    assert.equal(_unusableInputEmissionCountForTests(), 0,
+      'absent last_activity must NOT emit a diagnostic (it is genuinely absent, not corrupt)');
+  });
+
+  test('well-formed last_activity emits nothing', () => {
+    _resetUnusableInputWarningsForTests();
+    const dir = track(makeProject({
+      state: makeStateWithActivity('2026-06-13T12:00:00Z'),
+    }));
+    detectSignals(dir);
+    assert.equal(_unusableInputEmissionCountForTests(), 0,
+      'well-formed last_activity must NOT emit a diagnostic');
+  });
+
+  test('unusable last_activity does not re-emit on second call (dedup)', () => {
+    _resetUnusableInputWarningsForTests();
+    const dir = track(makeProject({
+      state: makeStateWithActivity('gibberish'),
+    }));
+    detectSignals(dir);
+    detectSignals(dir);
+    assert.equal(_unusableInputEmissionCountForTests(), 1,
+      'dedup: second call on the same source must not re-emit');
+  });
+});

tests/unusable-input.test.cjs

@@ -72,7 +72,7 @@ describe('UNUSABLE_REASON', () => {
     // (enum + call site + this assertion) instead of a silent widening.
     assert.deepStrictEqual(
       Object.keys(UNUSABLE_REASON).sort(),
-      ['FRONTMATTER_UNTERMINATED', 'ROADMAP_UNREADABLE'],
+      ['FRONTMATTER_UNTERMINATED', 'LAST_ACTIVITY_UNPARSEABLE', 'ROADMAP_UNREADABLE'],
     );
     assert.strictEqual(UNUSABLE_REASON.FRONTMATTER_UNTERMINATED, 'frontmatter_unterminated');
   });