fix(tasks): producer diagnostics name the seam, not one implementation

Review feedback (tianyicui, PR #657 inline): the missing-service message
should mention dsh-tasks, which defines ctx.tasks, rather than promoting a
specific backend. The seam's own surfaces (README, the direct-mount fence)
keep pointing at implementations, so the pointer chain still lands on
dsh-tasks-local without the producer strings going stale when another
backend becomes the recommended default. Agent Note updated accordingly
(en+zh, re-recorded).

packages/pty/tool-pty/README.md

@@ -66,4 +66,4 @@ Append-only; new results follow the reusable request prefix.
 ## Known Limitations and Deferred Work
 
 - No named key sequence, TUI, BEL, resize, auto-start, or cross-agent sharing schema is exposed.
-- Background mode requires both `@deepseek-ai/dsh-tasks-local` and the model-facing control surface from `@deepseek-ai/dsh-tool-tasks`.
+- Background mode requires both `@deepseek-ai/dsh-tasks` and its model-facing control surface.

packages/pty/tool-pty/src/index.ts

@@ -250,7 +250,7 @@ export function apply(ctx: Context, config: Config = {}): void {
       if (args.run_in_background === true) {
         if (!enableRunInBackground) throw new Error('background terminal sends are disabled by tool-pty configuration')
         const tasks = ctx.get('tasks')
-        if (tasks === undefined) throw new Error('background terminal sends require @deepseek-ai/dsh-tasks-local and @deepseek-ai/dsh-tool-tasks')
+        if (tasks === undefined) throw new Error('background terminal sends require @deepseek-ai/dsh-tasks and @deepseek-ai/dsh-tool-tasks')
         let cancelRequested = false
         const taskId = tasks.start({
           kind: 'pty-send',