fix(unified-sessions): stop the firstPrompt workingDir backfill from cross-contaminating history rows

COD-140's backfill was meant to cover live/persisted rows whose Codeman
id doesn't match an on-disk transcript UUID, guessing from the newest
transcript in the same workingDir as a last resort. It was also firing
for pure history rows whose OWN transcript scan already ran (and
genuinely found nothing, e.g. an oversized first message) -- those got
silently backfilled with the newest OTHER session's opening line from
the same directory. Not a blank row, but actively wrong: old sessions
displayed a completely unrelated (often today's live) conversation's
first prompt as if it were their own.

Skip the workingDir guess for any item that already has its own
'history' source -- it already had a real, direct attempt. Rows with
no history source at all (their transcript isn't linked/scanned under
their own id yet) still get the guess, matching the original intent.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

test/services/unified-session-service.test.ts

@@ -270,6 +270,39 @@ describe('mergeUnifiedSessions', () => {
     expect(live!.firstPrompt).toBeUndefined();
   });
 
+  it('does NOT borrow a sibling transcript for a history-only row whose own extraction failed (no cross-contamination)', () => {
+    // A pure history row already got its own real scan (step 1 keys it under its
+    // OWN sessionId) — if that extraction genuinely failed (oversized first
+    // message, noise-filtered, etc.), the workingDir guess must not paper over
+    // it with an unrelated session's opening line. Regression: an old session
+    // in a shared workingDir was displaying TODAY's live session's firstPrompt
+    // as its own, because the guess didn't check whether this row already had
+    // its own (failed) attempt.
+    const merged = mergeUnifiedSessions({
+      history: [
+        // This session's own transcript scan found no usable prompt.
+        {
+          sessionId: 'old-uuid',
+          workingDir: '/shared',
+          sizeBytes: 5000,
+          lastModified: '2026-01-01T00:00:00.000Z',
+          firstPrompt: undefined,
+        },
+        // A much newer, unrelated session in the same directory.
+        {
+          sessionId: 'newer-uuid',
+          workingDir: '/shared',
+          sizeBytes: 6000,
+          lastModified: '2026-06-01T00:00:00.000Z',
+          firstPrompt: "today's real prompt",
+        },
+      ],
+    });
+    const old = merged.find((m) => m.sessionId === 'old-uuid');
+    expect(old).toBeDefined();
+    expect(old!.firstPrompt).toBeUndefined();
+  });
+
   // COD-145: lastPrompt backfill — mirrors the COD-140 firstPrompt path so the
   // most-recent user prompt also reaches live rows whose id ≠ transcript UUID.
   it('backfills lastPrompt onto a live session by claudeSessionId join (uuid-join)', () => {