perf: rest animations, cheaper chat typing and server polls, async per-tool hooks

UI
- No animation runs forever: Sagi moods, the custom pet sprite, status dots
  (pulse-dot, pulse-slow, ping) and the chat shimmer play a few cycles and
  rest. An infinite animation kept the browser drawing 60 frames a second.
- useSagiBrain: frames that only move lastActivityAt by <1 s keep the old
  state; the 1 s tick re-renders only when the mood changes.
- ConversationView and MessageList are memoized and PaneChat passes stable
  callbacks: typing in the chat box no longer re-renders the conversation.
- The chat's 700 ms menu poll reads plain text first and scans the terminal
  cell by cell (reusing one cell) only when a row could be a menu cursor row.

Server
- GET /sessions/:id/transcript?after=N skips lines <= N before JSON.parse.
- GET /sessions/:id/outline is cached until the session's events change.
- broadcastLane uses the cached tmux.liveSessions(); tmux.hasSession answers
  "yes" from the fresh shared list instead of a synchronous spawn (50-150 ms
  per psmux call on Windows, blocking terminal echo).
- pane-mode polls every 150 ms; writeHourly writes only changed hour buckets.
- /api/metrics: agenthub_event_loop_delay_seconds{quantile}.

Hooks
- PreToolUse/PostToolUse run with "async": true (settings installer,
  plugin.json, S3 managed settings): Claude Code no longer waits a Node start
  before and after every tool call.

Committed with --no-verify at the owner's request: the pre-commit full server
suite fails on 9 tests that fail identically on 0c2f498 (lane-sync merge: no
git identity on this host; read-pool; resolveHost; codex-usage 1 ms flake).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/MONITORING.md

@@ -24,7 +24,8 @@ as they are into an existing Grafana (Dashboards → New → Import).
 ## Metrics used
 
 `agenthub_up`, `agenthub_build_info{version}`, `agenthub_process_uptime_seconds`,
-`agenthub_process_resident_memory_bytes`, `agenthub_sessions{status}`,
+`agenthub_process_resident_memory_bytes`, `agenthub_event_loop_delay_seconds{quantile}`,
+`agenthub_sessions{status}`,
 `agenthub_agents{status}`, `agenthub_events_total`, `agenthub_websocket_clients`,
 `agenthub_remote_sources{enabled}`, `agenthub_tokens_total{kind}`.