feat(acp): multiplex N concurrent ACP sessions + bash task ownership (RFC 011)

Lifts the RFC 010 single-session-per-connection cap: the bridge now runs N
concurrent sessions over one connection, each mapped to its own LoopAgent.

- packages/acp: live sessions held in a Map<sessionId, SessionRecord> with an
  agent→sessionId reverse WeakMap so agent/* events (which carry only the
  Agent) demux in O(1). Every session/event and agent/status is routed strictly
  to its owning record — concurrent sessions never cross-settle or interleave
  their session/update notifications. Per-session state: one in-flight prompt
  each, session/cancel aborts+settles only its own agent/prompt, session/load
  reserves a per-id load slot (distinct ids load concurrently; re-loading a live
  id is rejected), and disposal drains every live session in parallel to
  quiescence.
- packages/tool-bash: record each background task's owning agent at spawn and
  keep it for the executor's lifetime (NOT cleared on completion).
  bash_output/bash_kill reject a task owned by a different agent (a task with no
  owner is open; a no-agent caller can't access an owned task). Task ids are
  global and predictable, so this is the fence that stops one session's agent
  from reading/killing another session's background task.
- Per-session permission ownership and a per-agent disposer seam stay deferred
  (depend on the deferred permission gate); the reverse map the gate will route
  through is in place. RFC 011 stays `proposed`.
- Tests: two sessions stream concurrently without interleave; cross-session
  cancel isolation; per-session in-flight enforcement; dispose-all-to-quiescence;
  bash cross-session read/kill rejected (+ no-agent and unowned-task cases).
- Docs: RFC 011 implementation-status note; acp + tool-bash READMEs; example
  MVP-limitations updated. 100% per-file coverage maintained.

docs/rfc/011-acp-multi-session.md

@@ -2,6 +2,8 @@
 
 Status: proposed
 
+> **Implementation status:** the multi-session bridge (steps 1, 3, 4) and the bash task-ownership isolation are implemented in `packages/acp` + `packages/tool-bash`. **Per-session *permission* ownership is deferred** — it depends on the RFC 010 permission gate (`TODO(rfc010-permission-gate)`), which is itself deferred; the `agent→sessionId` reverse map the gate will route through is in place. Step 2's "real per-session disposer scope" is also deferred (`TODO(rfc010-agent-disposal)`): the bridge demuxes via id-keyed maps and global `ctx.on` listeners (correct and leak-free — disposal drains every session in parallel to quiescence), and a per-agent disposer seam is the follow-up. Status stays `proposed` until per-session permission ownership lands.
+
 ## Problem
 
 RFC 010 ships ACP support with a single active session per connection: a second `session/new` is rejected. Editors expect to run several conversations over one agent subprocess — a user opens multiple threads, or a client pre-warms sessions. The single-session guard is a deliberate MVP scope cut, not an architectural limit; this RFC lifts it.