docs(acp,rfc): fix stale ownership wording + propose unifying agent/session id (review)

Review follow-ups on the bash owner-token PR:

- packages/acp/README.md still described task isolation in object-identity terms
  ("records each background task's owning agent", "a different agent"). Rewrite
  to the session-token model: ownership is by `session.header.id`, stored on the
  executor's task, so a different Agent object on the same session may access it
  and ownership survives a tool-bash HMR reload.

- The reviewer flagged that the notice routes by `session.header.id` while the
  registry only enforces unique `agent.id`, so a programmatic caller could
  register two agents sharing a session token and mis-route a notice (not
  reachable via ACP). Rather than bolt a session-id invariant onto the generic
  registry, add a proposed RFC (2026-06-20-unify-agent-and-session-id) to remove
  the precondition by construction — an agent IS its session, one id — with a
  full risks discussion (forecloses multi-session-actor / fork futures, makes the
  config resume-or-create policy load-bearing, migration churn). The actual
  unification ships as its own Codex-converged PR. Cross-linked from the
  agent-lifecycle RFC's seam-precondition note.

- Reframe the tool-bash module-doc ownership paragraph to current-state (per the
  new AGENTS.md doc convention): contrast storing the token on the executor vs
  in the plugin as a standing rationale, not as "closing the old gap".

packages/acp/README.md

@@ -34,7 +34,7 @@ It is a **client-driver / UI plugin**, the structured analogue of the readline `
 
 The bridge multiplexes N sessions over one connection. Live sessions are held in a `Map<sessionId, SessionRecord>` (forward) with a `WeakMap<Agent, sessionId>` reverse map 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, so concurrent sessions never cross-settle or interleave their `session/update` notifications. State is per session: one in-flight prompt each, `session/cancel` aborts and settles only its own agent/prompt, and disposal drains every live session in parallel to quiescence. (Per-session *permission* ownership is reserved for the deferred permission gate — `TODO(rfc010-permission-gate)`.)
 
-Background-task isolation rides on `dsh-tool-bash`: bash task ids are global and predictable, so the tool layer records each background task's owning agent and `bash_output`/`bash_kill` reject a task owned by a different agent — one session's agent can't read or kill another's task.
+Background-task isolation rides on `dsh-tool-bash`: bash task ids are global and predictable, so each task carries an opaque owner token — the owning agent's `session.header.id` — stored on the task inside the executor (`dsh-bash`'s `ownerOf(id)` seam). `bash_output`/`bash_kill` reject a task whose token differs from the caller's session token, so one session's agent can't read or kill another's task. Ownership is by session TOKEN, not `Agent` object identity — a different `Agent` object on the same session may access the task — and because the token lives on the executor's task it survives a `tool-bash` HMR reload.
 
 ## Per-session cwd