simplify(seams): prune dead methods from the persistence and bash seams

Two capability seams carried abstract methods no production consumer calls.
A method no consumer programs against is not a seam — it is speculative
surface every implementation must still provide and test.

- SessionPersistence: remove has() and delete(), the coordinator's
  has/delete/deleteCore, and the PersistenceBackend.deleteStored hook (with its
  jsonl + sqlite + in-spec memory-stub impls). Surviving service surface:
  create/append/load/list. Production uses only load() (resume) and list()
  (ACP session/list).
- BashExecutor: remove get(id) and list(), the abstract decls and the
  LocalBashExecutor impls. The internal tasks map survives (it backs
  ownerOf/readOutput/kill); get/list were pure public accessors over it with no
  shipping caller and no bash_list tool.
- Migrate tests that reached through ctx.bash.get(id) to the public completion
  seam: a doneFor(id) helper over onTaskDone awaits a task by id, and the
  HMR-reload ownership test now proves task survival through A's own bash_output
  ([status: running]) plus ownerOf + B-rejection — a stronger through-the-tool
  assertion than the removed lookup peek.
- Update seam READMEs (six -> four service methods, drop the deleteStored hook
  and the get/list row) and the two implemented persistence RFCs in place.

Implements docs/rfc/implemented/simplification/2026-06-20-prune-dead-seam-methods.md

packages/bash/bash-local/tests/executor.spec.ts

@@ -98,8 +98,6 @@ describe('LocalBashExecutor background tasks', () => {
     const task = bash.start(bash.resolve({ command: 'sleep 0.2; echo done' }))
     expect(Date.now() - before).toBeLessThan(150)
     expect(task.status).toBe('running')
-    expect(bash.get(task.id)).toBe(task)
-    expect(bash.list()).toContain(task)
     await task.done
     expect(task.status).toBe('completed')
     expect(task.exitCode).toBe(0)
@@ -237,7 +235,6 @@ describe('LocalBashExecutor background tasks', () => {
     await running.done
     expect(finished.status).toBe('completed')
     expect(running.signal).toBe('SIGTERM')
-    expect(bash.list()).toEqual([])
   })
 
   it('disposing the executor fiber kills running tasks (no orphans)', async () => {
@@ -249,14 +246,13 @@ describe('LocalBashExecutor background tasks', () => {
     bash.onTaskDone(listener)
 
     const task = bash.start(bash.resolve({ command: 'sleep 60' }))
-    const running = bash.get(task.id)!
+    const running = task
     await new Promise(resolve => setTimeout(resolve, 50))
 
     // Grab the pid before dispose clears the registry.
     const pid = (running as unknown as { running: { pid: number } }).running.pid
     await fiber.dispose()
     await waitGone(pid)
-    expect(bash.list()).toEqual([])
     // Listener silenced by base-class teardown — no late notifications.
     expect(listener).not.toHaveBeenCalled()
   })