fix(tmux): merge-time fixes for the exited-agent report (#466)

- docs/wiki/The-Dashboard.md: the tab-appearance table gains the exited
  state (muted dot plus an `exited (137)` badge) and explains the bare
  `exited` variant.
- The detailed sidebar and rail no longer pair the muted dot with an "idle"
  pill: an exited session's pill reads "exited" (neutral styling) and its
  since stamp measures from the observed exit. This is a label override on
  the row model, not a new state, so SESSION_ACTIVITY_RANK and the home
  screen order are untouched, and a pending alert still keeps its own pill.
  The row signature includes the flag so the incremental path repaints it.
- The exited badge is aria-hidden like its sibling badges, and the exit is
  appended to the tab's aria-label in both render paths through one helper.
- test/tmux-manager.test.ts re-adds the junk-trailing-field parser case
  against parsePaneRows.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/session-sidebar-ux.test.ts

@@ -84,7 +84,7 @@ describe('watching badge on a rich session row', () => {
     // pill that says which one still decides the row; this badge only adds a fact.
     const meta = app.slice(app.indexOf('_sidebarRichMetaHTML(row) {'));
     const body = meta.slice(0, meta.indexOf('_sidebarRichStampText(timestamp, format) {'));
-    expect(body).toContain('tab-pill tab-pill--${escapeHtml(row.state)}');
+    expect(body).toContain('tab-pill tab-pill--${escapeHtml(pillMod)}');
     expect(body).toContain('tab-pill tab-pill--watching');
     expect(body).toContain('Still running in the background:');
   });
@@ -93,7 +93,7 @@ describe('watching badge on a rich session row', () => {
     // The meta line is rebuilt only when this signature moves, so a badge left out of
     // it would appear and disappear a render late, or not at all.
     expect(app).toContain(
-      'const sig = `${row.state}:${row.since ? row.since.at : 0}:${row.createdAt}:${row.watching}`'
+      "const sig = `${row.state}${row.exited ? '+exited' : ''}:${row.since ? row.since.at : 0}:${row.createdAt}:${row.watching}`"
     );
   });