feat(events): reconnect with last_event_id — replay missed events instead of losing them silently #14818

Part of #14815.

Problem

Events published while a client is disconnected are lost silently. There is no gap detection
anywhere, so the failure is invisible to both the user and the logs.

Current state:

• autobot-frontend/src/services/LiveEventService.ts:218 reconnects with exponential backoff and
  correctly re-subscribes its channels on reopen (:148).
• autobot-frontend/src/services/GlobalWebSocketService.ts:447 reconnects with backoff plus a
  /api/health polling wait mode.
• Neither sends a last-seen event id, and api/live_events.py has nowhere to receive one.
• LiveEventService.ts:409 receives event_id and only logs it — no high-water mark, no gap check.

This is the pattern AHP specifies as reconnect { clientId, lastSeenServerSeq, subscriptions[] },
answered with either replayed actions or a fresh snapshot when the gap exceeds the buffer
(AHP docs/specification/lifecycle.md).

Scope

Backend — autobot-backend/api/live_events.py:_handle_message

• Accept an optional last_event_id on the subscribe action.
• If the id is within the retained stream, replay the missed events via xrange before resuming
  live delivery.
• If the id is older than maxlen (or unknown), respond with a resync signal instead of a partial
  replay — never a silent partial.
• Mirror AHP's missing[]: name any requested channel that cannot be resumed.

Frontend — autobot-frontend/src/services/LiveEventService.ts

• Track the highest event_id seen per channel.
• Send it on re-subscribe after a reconnect.
• On a resync signal, clear local state for that channel and refetch rather than appending onto a
  stale view.

Depends on #14816 and #14817 — replay needs a durable stream and a durable id.

Acceptance criteria

• Events published while a client is disconnected are delivered on reconnect.
• A gap larger than the retention window produces an explicit resync, never a silent hole.
• Test: publish N events with the socket closed, reconnect, assert all N arrive in order.
• Test: publish more than maxlen events, reconnect, assert a resync signal rather than a
  partial replay.

Sources

Parent: #14815 · Full analysis: docs/research/agent-host-and-agent-client-protocols.md

• Agent Host Protocol — Microsoft, MIT — https://microsoft.github.io/agent-host-protocol/ ·
  https://github.com/microsoft/agent-host-protocol
• Agent Client Protocol — Zed + JetBrains, Apache-2.0 —
  https://agentclientprotocol.com/get-started/introduction ·
  https://github.com/zed-industries/agent-client-protocol