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/tool-bash/src/index.ts

@@ -22,10 +22,11 @@
  * multi-session ACP (RFC 011) this token check is the fence that stops one
  * session's agent from reading or killing another session's background task.
  *
- * Because ownership lives on the task in the EXECUTOR (disposed with the
- * `dsh-bash` fiber), it SURVIVES a `tool-bash` HMR reload — closing the old
- * plugin-local-map gap where a reload orphaned pre-reload tasks. (The
- * `onTaskDone` listener is still effect-scoped to this plugin's `apply`, so a
+ * Storing the token on the task in the EXECUTOR (disposed with the `dsh-bash`
+ * fiber), rather than in this plugin, is what makes ownership survive a
+ * `tool-bash` HMR reload — a reload that reset a plugin-local map would orphan
+ * a task spawned before it. (The `onTaskDone` listener is still effect-scoped
+ * to this plugin's `apply`, so a
  * completion landing during the reload gap still drops its one notice — the
  * pre-existing reload-gap drop — but the ownership fence itself is HMR-proof.)
  *