feat(hooks): Notification event — fires when agent needs user attention

[emal-avala]

Summary

Adds a Notification hook event. Previously the sink.on_warning path only reached stderr (and the JSONL warning event as of #215); users couldn't wire a desktop notifier, Slack DM, or SMS to those moments because there was no hook.

Context payload

{
  "session_id": "...",
  "message": "...",
  "title": "..." | null,
  "notification_type": "budget_limit" | "context_full" | ...
}

notification_type lets hook configs filter so a Slack-on-budget hook doesn't also fire on routine context warnings.

Wiring

• HookEvent::Notification added with snake_case serde
• fire_notification_hooks(message, title, notification_type) async method on QueryEngine
• Wired at the 3 most critical user-attention sites in run_turn_with_sink:
  • BudgetDecision::Stop → "budget_limit"
  • BudgetDecision::ContinueWithWarning → "budget_warning"
  • Context window is_blocking → "context_full"
• Non-blocking context-% warnings and per-turn rate-limit fallbacks stay warning-only — routing every retry would be noise
• HOOK_EVENT_CATALOG, format_hook_event, parse_hook_event updated
• hooks/mod.rs module docs updated

Test plan

• cargo clippy --workspace --tests --no-deps -- -D warnings — clean
• cargo test -p agent-code-lib --lib hooks::tests — 6 pass
• cargo test -p agent-code-lib --lib config::schema::tests::hook_event_serde_roundtrip_notification — pass
• cargo test -p agent-code --bin agent commands::tests::parse_hook_event — 7 pass

3 new tests:

1. Schema serde round-trip (notification ↔ "notification")
2. run_hooks dispatches for Notification
3. parse_hook_event accepts "notification" / "Notification"

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