feat(ui): Stop button + queue-at-boundary for task interrupt (AGX1-391)

[declan-scale]

Draft. Frontend half of AGX1-391 for the native agentex chat. Split out of the backend PR #365.

Included

• Dual-mode composer button: Stop when a turn is streaming and the input is empty; Send (which enqueues) when there is text; normal Send when idle.
• Esc-to-interrupt while streaming.
• Queued-message chip, flushed on the streaming->idle edge (queue-at-boundary).
• INTERRUPTED treated as non-terminal (composer stays enabled; running is judged from rpcStatus).
• Sync message/send AbortController exposed so Stop tears the stream down; meta cache settled to idle so no hung 'thinking' indicator.

Depends on

• scale-agentex#365 (the POST /tasks/{id}/interrupt route + INTERRUPTED status). The interrupt call uses a thin REST adapter with a TODO(AGX1-391) to swap to the typed SDK method after regeneration.

Note

• A couple of explanatory comments were added in the .ts hooks; will strip to match the no-inline-comment convention before marking ready.

🤖 Generated with Claude Code

[declan-scale]

Closing per design discussion on AGX1-391: interruption must be implemented by the agent itself (the agent stops its own turn and sets its state to INTERRUPTED; the control plane just forwards task/interrupt like event/send). A generic Stop button in the native agentex-ui would be misleading for any agent that does not implement self-interruption. The golden-agent-specific Stop + queue-at-boundary UI stays in scaleapi#151564 since the golden agent implements the interrupt handler.