dashboard: session picker, completion detection, lifecycle fixes

Web dashboard improvements:
- Command-palette session navigator: click session name to open
  searchable picker with active/paused/archived grouping
- Completion detection: sessions with all agents disconnected and
  in terminal states show "completed" badge with frozen timer
- Auto-switch to newer active session (disabled when user pins one)
- Poll generation tracking prevents stale loops on session switch

Bug fixes:
- broker: clean up dangling slot references when peer record gone
  but slot still points to it (zombie/race condition)
- e2e: Codex-Tester now calls approve(target=Claude-Engineer) per
  proper QA workflow; e2e completion requires actual approved state
- e2e cleanup: unregister orphaned MCP server peers on teardown