Implement cancellation token propagation for ACP sessions

[bug-ops]

Parent Epic

#777

Problem

cancel() is currently a no-op. The agent loop continues running after IDE sends cancel, wasting resources and blocking the session.

Solution

• Add CancellationToken (tokio-util) to SessionEntry
• Create token on new_session, store in session map
• cancel() calls token.cancel()
• Agent loop checks token.is_cancelled() between LLM calls and tool executions
• prompt() drain loop selects on token cancellation, returns StopReason::Cancelled
• Wire through LoopbackChannel or as separate signal

Critical Path

Depends on: #779 (AcpContext wiring — agent loop must be ACP-aware)
Blocks: nothing directly, but required for responsive IDE UX

Acceptance Criteria

• CancellationToken stored per session
• cancel() triggers token
• Agent loop exits promptly on cancellation
• prompt() returns StopReason::Cancelled
• Integration test: prompt → cancel → verify cancelled response