Fix multi-slot prompt queue

[qchiujunhao]

Summary

• Fixed issue Orbit: multi-slot prompt queue (stack thoughts while agent works) #60
• replace the single pending prompt slot with a FIFO queue for prompts submitted while the agent is streaming
• add queued-message UI with collapse, remove, and clear controls
• preserve immediate handling for local slash commands while queuing agent-backed slash commands until the current turn ends

Verification

• git diff --check
• npm run typecheck
• cd app && npm exec tsc -- --noEmit
• manual fake-agent browser review for multi-message queue, row removal, clear, error, and FIFO flush behavior
• manual fake-agent browser review for /help immediate handling plus queued /plan and /summarize behavior

[dannon]

Nice, this is a solid upgrade from the single-slot version -- the FIFO refactor reads well and the local-vs-agent slash split is a nice touch.

Two things before merge:

• Stop doesn't actually stop. abort/Escape dropped the old clearPendingMessage(), so now hitting Stop aborts the turn -> agent_end fires -> flushNextQueuedMessage() runs -> the next queued prompt immediately kicks off. So with a queue, Stop just skips to the next one. Probably want clearQueue() back on abort + Escape.

• /cost is missing from LOCAL_SLASH_COMMANDS. It's purely local, so right now it gets queued mid-stream instead of running right away. One-line add. (Side note: that set is a hand-kept duplicate of the no-prompt branches in handleSlashCommand and it's already drifted -- might be worth driving it off a flag instead so it can't.)

Rest is minor: no tests on the queue helpers, and the aria-live panel re-announces the whole queue on every change. Not blockers.

Mostly just want the Stop behavior sorted before this goes in.

[qchiujunhao]

Nice, this is a solid upgrade from the single-slot version -- the FIFO refactor reads well and the local-vs-agent slash split is a nice touch.

Two things before merge:

• Stop doesn't actually stop. abort/Escape dropped the old clearPendingMessage(), so now hitting Stop aborts the turn -> agent_end fires -> flushNextQueuedMessage() runs -> the next queued prompt immediately kicks off. So with a queue, Stop just skips to the next one. Probably want clearQueue() back on abort + Escape.
• /cost is missing from LOCAL_SLASH_COMMANDS. It's purely local, so right now it gets queued mid-stream instead of running right away. One-line add. (Side note: that set is a hand-kept duplicate of the no-prompt branches in handleSlashCommand and it's already drifted -- might be worth driving it off a flag instead so it can't.)

Rest is minor: no tests on the queue helpers, and the aria-live panel re-announces the whole queue on every change. Not blockers.

Mostly just want the Stop behavior sorted before this goes in.

Thank you @dannon ! changes made.

[dannon]

Looks great -- both blockers handled (Stop clears the queue now, /cost no longer waits), and the queue-helper tests are a nice add. Routing /cost through a followUp instead of a local append is the right call too; keeps the agent as the single writer of notebook.md and avoids a write race. Pulled it down and confirmed the new tests and app typecheck both pass. Thanks for the quick turnaround!