feat(acp): stream shell output as v2 terminals

[danielkov]

Summary

Stream local shell stdout and stderr as ACP v2 terminal output and attach the terminal to its existing projected inner tool call. Preserve the structured shell result and keep ACP v1 unchanged.

Closes #154.

Technical details

Terminal IDs reuse ToolRequest.call_id. Independently base64-encoded chunks preserve arbitrary bytes. Separate bounded protocol queues exclude terminal traffic from v1 before admission; Observed remains the sole owner of call lifecycle, and session finalization retains its projection drain barrier.

Each session projection subscription admits at most 128 output chunks and a cumulative 1 MiB of encoded output and optional command/cwd metadata. The budget persists across calls and turns, bounding streamed payloads even when the SDK's outgoing queue stalls. Further output carries kit/outputIncomplete metadata; terminal exits and call completion are not charged against the preview budget. Structured shell capture is unchanged.

Normal completion reports the shell exit code. Cancellation, timeout, and errors mark the terminal exited before the call ends without inventing an exit code. Projection bus loss exits visible terminals with kit/outputIncomplete metadata before invalidating their cards.

[kit-code-agent]

Terminal streaming can invalidate ACP v1 tool cards under output bursts and accumulate output in unbounded ACP v2 transport queues for slow clients.

[kit-code-agent]

No issues found. The changes look good to merge.