revert bash get()/list() removal — keep persistence-only prune

The original prune removed BashExecutor.get()/.list() too, but each is a
one-line accessor over the executor's already-tracked tasks map, and removing
them forced dsh-tool-bash's tests onto a ~35-line onTaskDone completion-tracking
harness just to replace the one-line ctx.bash.get(id) lookup. Per the AGENTS.md
"RFCs are proposals, not golden truth" principle, that disproportionate
migration cost is evidence the methods earn their keep — a test harness IS a
consumer programming against the seam.

Restore get()/list() (seam + LocalBashExecutor impl + the bash tests that used
them, dropping the doneFor/trackCompletions scaffolding). The persistence
has()/delete()/deleteStored removal stands — it had only contract-test callers
and no test-ergonomics cost. The RFC is retitled persistence-only with an
implementation note recording the bash revert.

packages/bash/bash/tests/service.spec.ts

@@ -44,10 +44,18 @@ 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}"`)
@@ -80,6 +88,8 @@ 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' }))