feat(mcp): expose Agent event stream and batch runtime status

[JoyaWang]

Summary

This PR adds an event-first status path for remote/local clients without changing the existing MCP POST contract or renderer IPC contract.

• Add authenticated loopback GET /events SSE endpoint.
• Forward the existing normalized AgentEventEnvelope from local sidecar, native-agent, and remote-host event paths.
• Add bounded session filtering, subscriber limits, disconnect cleanup, and heartbeat frames.
• Add agent/getStatuses and pi_agent_status_batch for one-call initial/reconnect snapshots.
• Keep all existing permission/lifecycle validation on the Electron-main IPC path.
• Update English and Chinese IPC specs and register E2E-221.

Why

The current mobile client must poll pi_agent_status and pi_session_get. That creates per-session RPC fan-out, adds visible latency, and loses intermediate retry/error/progress events. The new contract is subscribe-first, snapshot-on-start/reconnect, with polling only as a compatibility fallback for older transports.

Validation

• pnpm --filter @pi-desktop/agent-runtime build — PASS
• pnpm --filter @pi-desktop/shared build — PASS
• pnpm --filter @pi-desktop/desktop typecheck — PASS
• pnpm lint:biome — PASS
• pnpm docs:check — PASS (81 English/Chinese pairs, 517 docs pages)
• node --test apps/desktop/test/agent-events.test.mjs apps/desktop/test/mcp-control.test.mjs — 11/11 PASS
• pnpm check:pr-base — PASS

Full Electron E2E was not run in this environment; E2E-221 documents the required candidate validation and remaining risk.

Refs: mocode #467

[JoyaWang]

@vastsa — gentle ping on this one: it has been open since 2026-09-27 with all four CI jobs green and no review comments yet, so I suspect it may have slipped off the radar.

Where it stands:

• feat(mcp): expose Agent event stream and batch runtime status — 13 files, +450/−27, head 2b9e25d95
• CI: Docs checks / Head contains latest base / JS build+typecheck+lint+architecture+test / Rust host-core — all pass
• Merge state was CLEAN at last check

It is the natural companion to #1014 (which exposes pending asktool questions): this one adds the Agent event stream plus a batched runtime-status read, so a remote client can follow a turn without polling per-entity calls.

If it needs a rebase onto the current main — main has moved a few hundred commits since this was opened — say the word and I will rebase, resolve, and re-run the same validation set I used for #1014. If the design needs changes, a review with the specific concerns is more useful to me than silence, and I am happy to iterate.

Thank you.