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/tool-bash/tests/integration.spec.ts

@@ -8,6 +8,7 @@ import ToolRegistry from '@deepseek-ai/dsh-tools'
 import AgentRegistry from '@deepseek-ai/dsh-agent'
 import AgentLoop, { ReactLoopAgent } from '@deepseek-ai/dsh-agent-loop'
 import { LocalBashExecutor } from '@deepseek-ai/dsh-bash-local'
+import type { BashTask } from '@deepseek-ai/dsh-bash'
 import * as ToolBash from '@deepseek-ai/dsh-tool-bash'
 import { MockAdapter, textResponse, toolCallResponse } from '../../../core/agent-loop/tests/mock-adapter.ts'
 
@@ -143,13 +144,18 @@ describe('bash tool through the agent loop', () => {
       return next()
     })
 
+    // Capture the single background task's completion. Registered BEFORE send so
+    // a fast task (echo) can't finish before the listener is attached; onTaskDone
+    // delivers the task object once it completes (completion may race turn end).
+    const taskDone = new Promise<BashTask>((resolve) => {
+      const dispose = ctx.bash.onTaskDone((task) => { dispose(); resolve(task) })
+    })
+
     agent.send([{ type: 'text', text: 'run echo bg-ok in the background' }])
     await waitForIdle(ctx, agent)
 
     // Wait for the background task itself (completion may race turn end).
-    const task = ctx.bash.get(taskId)
-    if (!task) throw new Error(`task ${taskId} not registered`)
-    await task.done
+    await taskDone
 
     const log = events(agent)
     const firstResult = findEvent(log, 'tool/result')