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/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');
   });