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/src/index.ts

@@ -85,6 +85,9 @@ 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
@@ -100,6 +103,9 @@ 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