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/unusable-input.cts

@@ -49,6 +49,14 @@ const UNUSABLE_REASON = Object.freeze({
    * simply has no ROADMAP yet: absence returns the same sentinel, silently. (#1881)
    */
   ROADMAP_UNREADABLE: 'roadmap_unreadable',
+  /**
+   * A `last_activity` value in STATE.md frontmatter is present but unparseable as a date.
+   * Distinct from absent: absence means the field was never set, while an unparseable value
+   * means it was set to something Date.parse rejects. Per ADR-1411 amendment: corrupt is not
+   * absent — the fallback (stale_activity: false) stays, but a diagnostic is emitted so the
+   * silent degradation is visible. (#3099, sixth #1879 site)
+   */
+  LAST_ACTIVITY_UNPARSEABLE: 'last_activity_unparseable',
 } as const);
 
 type UnusableReason = (typeof UNUSABLE_REASON)[keyof typeof UNUSABLE_REASON];
@@ -59,6 +67,8 @@ const REASON_PROSE: Readonly<Record<UnusableReason, string>> = Object.freeze({
     'frontmatter opens with "---" but never closes; metadata was NOT applied',
   [UNUSABLE_REASON.ROADMAP_UNREADABLE]:
     'ROADMAP.md exists but could not be read; phase and milestone lookups fell back to defaults',
+  [UNUSABLE_REASON.LAST_ACTIVITY_UNPARSEABLE]:
+    'last_activity in STATE.md is present but unparseable as a date; stale_activity fell back to false (idle-stranded suppressed)',
 });
 
 // ─── Dedup state ──────────────────────────────────────────────────────────────