fix: sticky idle state prevents stale working status

[aterrylu]

Summary

• Agent sessions were showing as "working/spinning" in the dashboard even after finishing work
• Root cause: the deriveStatus() state machine had no protection against late-arriving hook events overriding the idle state
• Additionally, known CC bugs (#26770, #29881) mean Stop hooks sometimes don't fire, compounding the issue
• The "recap" feature (CC v2.1.108) was investigated but is unrelated — it's a session-resume summary, not end-of-turn output

Changes

• Sticky idle/stopped states — idle and stopped can only be exited by UserPromptSubmit, SessionStart, SessionEnd, PermissionRequest, Notification, or PreToolUse. Late PostToolUse, SubagentStop, etc. are logged and dropped
• Debug logging — dropped events now log with session ID prefix for diagnostics
• Dashboard default fix — changed ?? "working" fallback to ?? "unknown" in Sidebar and TabBar so agents with no hook state don't show as spinning
• Notification guard fix — Notification added to idle exit events so permission_prompt notifications can break through the sticky guard
• 11 new tests — comprehensive coverage for sticky idle behavior, sticky stopped, and transition edge cases

CC Bug References

• #26770 — Stop hooks via settings.json command type sometimes never fire
• #29881 — Stop hook only fires on session termination, not per-turn
• #40655 — Race: Stop hook fires before transcript JSONL flush

Test plan

• All 30 hook tests pass (16 existing + 11 new sticky + 3 notification)
• Biome lint/format clean
• Manual verification: spawn agent in dashboard, give it a task, verify status transitions correctly to idle after completion
• Verify permission prompts still show as "needs input" when agent is idle

🤖 Generated with Claude Code

[nox-0x]

Solid fix. Sticky idle/stopped guard is clean and well-tested (11 new cases). The UI fallback change from "working" to "unknown" is the right call — no hook state != actively working. LGTM.