Push agent status changes via per-pane event subscriptions

[powerfooI]

Follow-up to the multi-connection support in #18.

Problem

Agent running-status transitions (working/idle/done) had no push channel. Herdr emits pane.agent_status_changed only through per-pane parameterized subscriptions, which the bridge never made, so status sync relied solely on the 1s pane.list poll. Browsers throttle timers in hidden tabs, so an agent could stay showing a stale status (e.g. idle after starting work) until the tab regained focus.

Changes

• Bridge: new per-connection subscription loop (server/src/connections/agent-status-subscription.ts) that tracks panes with a detected agent. The pane set is seeded from pane.list and rebuilt on pane_agent_detected/pane_closed/tab_closed/workspace_closed with a debounced wake plus a fallback poll; membership changes requested mid-RPC are never lost, and transient subscribe failures retry without killing the loop.
• Bridge: HerdrClient.subscribe accepts parameterized subscriptions (e.g. { type, pane_id }).
• Web: accept dot-named events whose wire data has no type field (previously dropped by validation).
• Web: refresh metadata immediately when the page becomes visible, the window regains focus, or the network comes back online.

Verification

• bun run format, bun run lint, bun run typecheck (web + server)
• bun run test: 558 pass, including 10 new subscription-loop cases (rebuild races, retry, stop) and event-validation coverage
• Live end-to-end against herdr 0.8.0: status transitions reach the UI in ~0.1-0.5s instead of waiting for the next poll