Improve steering queue access

[markokocic]

Allow agent users to access and clear steering queue.

That would allow pi agent like queue handing in the client.

[yuanhao]

Thanks for the request! Two parts to this:

Clearing already works — clear_steering_queue(), clear_follow_up_queue(), and clear_all_queues() have been on Agent since v0.1.0.

Inspection was the missing half, and it's now implemented in #54:

• steering_queue_snapshot() / follow_up_queue_snapshot() — point-in-time copy for rendering the queue in a client (doesn't consume)
• steering_queue_len() / follow_up_queue_len()
• take_steering_queue() / take_follow_up_queue() — atomically drain and return
• steer_all() / follow_up_all() — batch requeue under a single lock, which is the primitive for pi-style edit-and-requeue:

let mut queued = agent.take_steering_queue(); // atomic drain
queued.pop();                                 // let the user edit/remove entries
agent.steer_all(queued);                      // requeue the survivors as one batch

A few semantics worth knowing for a queue UI (all documented in the PR): only the drain is atomic — if the run finishes while you're editing, requeued survivors are delivered at the start of the next run; messages the loop has already picked up are no longer in the queue and can't be retracted; and these accessors pair with the spawn-based prompt() flows (the _with_sender variants borrow the agent mutably for the whole run). We skipped per-index removal since it would race with the loop — take-all + steer_all is the atomic equivalent.

Would this cover your queue-handling use case? If something's missing for the pi-style client you have in mind, let us know before we cut the next release.

[yuanhao]

Released in v0.9.0 (via #54): steering_queue_snapshot() / follow_up_queue_snapshot(), *_len(), atomic take_*_queue(), and batch steer_all() / follow_up_all() for edit-and-requeue. The edit-window semantics (what is and isn't atomic) are documented in docs/concepts/agent-loop.md. Would love to hear if this covers your pi-style client — if something's missing, open a fresh issue.