feat(bash): add an opaque owner token to the executor seam

Background-task ownership needs a stable home that survives a consumer HMR
reload. Add an optional `owner?: string` to `BashExecRequest` and a
required-but-nullable `owner: string | undefined` to the resolved
`BashExecSpec` (mirroring how `workdir`/`timeoutMs` are required on the spec —
a forgotten owner is a visible `undefined`, never a silently-absent property
that yields an unowned, cross-session-readable task). `resolve()` carries it
through.

Expose the stored token via a new `BashExecutor.ownerOf(id): string |
undefined` seam (ONE read path — not also on the public `BashTask`). The
executor stores and returns the token verbatim and NEVER interprets it: the
access POLICY lives in the consumer (`dsh-tool-bash`). `bash-local` stores
`owner` on its `TrackedTask` and implements `ownerOf`; unknown-id and
known-but-ownerless both read as `undefined`. Because ownership lives on the
task in the executor (disposed with the `dsh-bash` fiber), it survives a
`tool-bash` HMR reload.

Updates the StubExecutor seam test and the bash/bash-local READMEs.

packages/bash-local/src/index.ts

@@ -49,6 +49,8 @@ interface TrackedTask extends BashTask {
   /** Whole-stream byte offsets already delivered via {@link LocalBashExecutor.readOutput}. */
   stdoutOffset: number
   stderrOffset: number
+  /** Opaque owner token from the {@link BashExecSpec} (the consumer's isolation key). */
+  owner: string | undefined
 }
 
 /**
@@ -114,6 +116,9 @@ export class LocalBashExecutor extends BashExecutor {
       workdir: request.workdir ?? this.config.cwd ?? process.cwd(),
       timeoutMs,
       ...request.signal ? { signal: request.signal } : {},
+      // Carry the owner through verbatim (required-but-nullable on the spec):
+      // the executor never interprets it — the consumer's access policy does.
+      owner: request.owner,
     }
   }
 
@@ -149,6 +154,7 @@ export class LocalBashExecutor extends BashExecutor {
       status: 'running',
       exitCode: null,
       signal: null,
+      owner: spec.owner,
       running,
       stdoutOffset: 0,
       stderrOffset: 0,
@@ -174,6 +180,12 @@ export class LocalBashExecutor extends BashExecutor {
     return this.tasks.get(id)
   }
 
+  ownerOf(id: string): string | undefined {
+    // Unknown id and known-but-ownerless both read as undefined — the consumer
+    // treats undefined as "open" and a truly unknown id fails at readOutput/kill.
+    return this.tasks.get(id)?.owner
+  }
+
   list(): BashTask[] {
     return [...this.tasks.values()]
   }