refactor(tool-bash): own background tasks by session token, not a plugin-local Map

Delete the `taskOwner: Map<string, Agent>` entirely — it served two roles
(access control AND holding a live Agent for completion notices), both now
stateless:

- Access control: `bash_output`/`bash_kill` compare `ctx.bash.ownerOf(id)` to
  the caller's token (`exec.agent?.session.header.id`) with `!== undefined`
  semantics (an empty-string token is still a real owner). The owner is stamped
  at spawn via `resolve({ …, owner })`. Ownership now lives on the task in the
  executor, so it SURVIVES a tool-bash HMR reload — closing the old
  XXX(tool-bash-owner-hmr) gap.
- Completion notice: `onTaskDone` reads `ctx.bash.ownerOf(task.id)` and finds
  the live agent by scanning `ctx.get('agents')?.list()` for a matching
  `session.header.id` (read via `ctx.get` — the listener runs on the bash
  fiber, a foreign fiber, where the `ctx.agents` proxy would throw). No
  registry / owner gone → drop the notice cleanly.

Token is `session.header.id` (NOT `session.id`): every other subsystem keys off
the header id, and the test fakes populate only `session.header.id`, so reading
`session.id` would make every fake unowned and pass the isolation tests for the
wrong reason.

Tests give A and B DISTINCT real session tokens (a same-token-different-Agent
case is now ALLOWED — identity no longer matters); the HMR test inverts to
assert ownership SURVIVES a tool-bash reload; a new test covers the
owner-agent-gone-before-completion drop. Migrates the agent-lifecycle RFC
proposed->implemented (recording all three seams + the session-id-uniqueness
precondition) and updates the tool-bash README + the now-implemented RFC's
cross-links.

packages/tool-bash/README.md

@@ -30,7 +30,7 @@ Result text: stdout, then a `[stderr]` section, then status markers — `[timed
 
 ### Task ownership (cross-session isolation)
 
-The owning agent is recorded per task id at spawn and kept for the lifetime of the loaded plugin instance (it is **not** cleared on completion). `bash_output`/`bash_kill` reject a task owned by a *different* agent with `task <id> belongs to another session` (a task started with no agent — a non-loop caller — has no owner and is open to anyone; a call with no `exec.agent` cannot access an owned task). Task ids are global and predictable, so under multi-session ACP this ownership check is the fence that stops one session's agent from reading or killing another session's background task. (`XXX(tool-bash-owner-hmr)`: an independent HMR reload of this plugin starts a fresh map, so a task spawned before the reload becomes un-owned — acceptable as HMR is dev-only and the session boundary is one user's cooperative editor; a durable fix attaches ownership to the executor/task lifetime.)
+The owning agent's session token (`session.header.id`) is stamped onto the task at spawn — passed to the executor via `resolve({ …, owner })` and stored ON THE TASK inside the executor (the `dsh-bash` `ownerOf(id)` seam), **not** in a plugin-local map. `bash_output`/`bash_kill` compare `ctx.bash.ownerOf(id)` to the caller's token (`session.header.id`) with `!== undefined` semantics and reject a task owned by a *different* session with `task <id> belongs to another session` (a task started with no agent — a non-loop caller — has no owner token and is open to anyone; a call with no `exec.agent` cannot access an owned task). Task ids are global and predictable, so under multi-session ACP 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 an independent `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 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.)
 
 ## UI presentation
 
@@ -38,7 +38,7 @@ These tools own how their calls render in a UI (an editor's tool-call card) via
 
 ## Background completion notices
 
-When a background task finishes, a short notice is injected into the owning agent's session (`agent.inject()`, source `{kind: 'plugin', plugin: 'tool-bash'}`). Injection is **durable context for the next model request, not a wake-up** — an idle agent stays idle until something sends a message. That's why the tool descriptions tell the model to poll with `bash_output`.
+When a background task finishes, a short notice is injected into the owning agent's session (`agent.inject()`, source `{kind: 'plugin', plugin: 'tool-bash'}`). The owning agent is found by its session token: the listener reads `ctx.bash.ownerOf(task.id)` and scans `ctx.get('agents')?.list()` for an agent whose `session.header.id` matches (read via `ctx.get` — `onTaskDone` runs on the bash fiber, a foreign fiber, so the `ctx.agents` proxy would throw). If no live agent carries that token — e.g. the owning session disconnected and its agent was disposed while the task ran on — the notice is dropped cleanly. Injection is **durable context for the next model request, not a wake-up** — an idle agent stays idle until something sends a message. That's why the tool descriptions tell the model to poll with `bash_output`.
 
 ## Permissions