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.

examples/acp-agent/README.md

@@ -32,4 +32,4 @@ Run from the repo root (the MVP requires the server's launch directory to be the
 
 ## MVP limitations
 
-The bridge is the RFC 010 MVP: single session per connection (RFC 011 lifts this), text-only prompts, `cwd` must equal the launch directory, and the tool-permission gate is deferred (`TODO(rfc010-permission-gate)` — tools run with the executor's full authority). See `packages/acp/README.md` for the full contract.
+The bridge supports N concurrent sessions per connection (RFC 011). Remaining limits: text-only prompts, `cwd` must equal the launch directory, and the tool-permission gate is deferred (`TODO(rfc010-permission-gate)` — tools run with the executor's full authority). See `packages/acp/README.md` for the full contract.