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/src/index.ts

@@ -85,9 +85,6 @@ export abstract class BashExecutor extends Service {
   /** Start a background task and return its handle immediately. */
   abstract start(spec: BashExecSpec): BashTask
 
-  /** Look up a background task by id. */
-  abstract get(id: string): BashTask | undefined
-
   /**
    * The opaque OWNER token recorded for a background task at {@link start}
    * (from the {@link BashExecSpec}'s `owner`), or `undefined` for an unknown id
@@ -103,9 +100,6 @@ export abstract class BashExecutor extends Service {
    */
   abstract ownerOf(id: string): string | undefined
 
-  /** All tracked background tasks (insertion order). */
-  abstract list(): BashTask[]
-
   /** Read output produced since the previous read. Throws for unknown ids. */
   abstract readOutput(id: string): BashTaskRead