Add cancellable SDK run controller

[victorxheng]

User request

A Freebuff cloud user reported that long-running terminal commands or autonomous agent work can get stuck for 20+ minutes, and requested a user-controlled way to terminate the running work.

Implementation

• Added CodebuffRunController and createRunController() in the SDK.
• Added CodebuffClient.runCancellable(), which returns { id, controller, signal, result, cancel } for hosts to keep in an in-flight run registry and call from a Stop/Terminate UI or API action.
• The cancel path composes with any caller-provided AbortSignal, then flows through the existing runtime cancellation path used by LLM streams, subagents, tool execution, and terminal commands.
• Exported the new controller and CancellableRun type from the SDK entrypoint.
• Added tests proving cancel() aborts an already-running agent signal and that external caller signals still abort the same run.

Confidence / verification

Ready to merge with high confidence for the SDK/runtime control surface. Verified locally:

• bun test sdk/src/__tests__/client-cancellable-run.test.ts sdk/src/__tests__/run-cancellation.test.ts
• cd sdk && bun run typecheck
• cd sdk && bun run build

Note: this public snapshot does not include the tracked cloud web route/component code for the Stop button itself; this PR provides the production SDK primitive that the cloud host can wire to its stop endpoint/UI without bypassing existing runtime cancellation semantics.