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/tests/service.spec.ts

@@ -44,18 +44,10 @@ class StubExecutor extends BashExecutor {
     return task
   }
 
-  get(id: string): BashTask | undefined {
-    return this.tasks.get(id)
-  }
-
   ownerOf(id: string): string | undefined {
     return this.owners.get(id)
   }
 
-  list(): BashTask[] {
-    return [...this.tasks.values()]
-  }
-
   readOutput(id: string): BashTaskRead {
     const task = this.tasks.get(id)
     if (!task) throw new Error(`unknown bash task "${id}"`)
@@ -88,8 +80,6 @@ describe('BashExecutor service seam', () => {
   it('registers as ctx.bash and serves the abstract API', async () => {
     const { bash } = await setup()
     const task = bash.start(bash.resolve({ command: 'sleep 1' }))
-    expect(bash.get(task.id)).toBe(task)
-    expect(bash.list()).toEqual([task])
     expect(bash.kill(task.id)).toBe(true)
     expect(bash.kill(task.id)).toBe(false)
     const result = await bash.run(bash.resolve({ command: 'true' }))