Agent Mode: optional mid-run steer alongside queue

[AnthonyRonning]

Summary

Maple Agent now has a staged FIFO queue: mid-run composer sends stay as chips and are promoted only after the current Goose Agent::reply finishes (the whole agentic loop). Some people would rather steer: inject follow-ups at the next model turn so the live loop can change course before it finishes every tool.

Queue is a strict improvement even for steer-preferring users. The workaround today is: queue chips, Stop, then Send. That works.

This issue is the follow-up to add steer as a second behavior, not to replace queue.

Why this is not a small flip

Goose already has Agent::steer (pinned 064244e). Maple used it in 14c62af8 and replaced it with the Maple-owned FIFO in fa85309c after race/QA issues.

Goose steer:

• does not interrupt the current completion or current tool batch
• drains pending steers at the next model turn of the same reply
• persists those messages immediately (metadata.steer = true)
• has no public edit-one / remove-one API
• keeps pending steers across cancel for resume

That is earlier than queue, but it is not a chip list. Chips, in-place edit, Discard, and Stop-leaves-chips assume Maple still owns the stack.

Proposed product shape

Keep queue as the default.

Add steer as an explicit second mode (setting TBD: per-task, per-account, or a composer control):

• Queue: wait for the current reply to finish; chips stay Maple-owned and editable.
• Steer: hand the follow-up to Goose at Send; the next model turn sees it; that item leaves the chip list and is no longer in-place-editable.

Do not silently make the current chips steer. ACP should stay out of this unless we explicitly take Goose's unstable _goose/unstable/session/steer.

Out of scope

• Replacing queue
• Interrupting an in-flight token stream or in-flight tool
• ACP steer unless product explicitly wants the unstable Goose wire method

Related: PR #781.

[AnthonyRonning]

I think the main question to answer here is:

1. Do we have both? Codex does.
2. Which is default?
3. How to surface switching between the two? Settings and/or something in the chat composer view?
4. Be able to support having both at the same time and doing one-off steer/queue

[AnthonyRonning]

Shipped the Maple-owned queue in #781.

This issue remains the follow-up for optional steer: inject follow-ups at the next Goose model turn instead of waiting for the whole agentic loop. Queue stays the default. Until then, steer-preferring users can queue, Stop, then Send.

[AnthonyRonning]

Draft implementation: #794