fix(grok): discard resume cursor when a thread's workspace changes

[justingray0]

Summary

A grok resume cursor is only valid against the working directory it was recorded in. When a thread moves to a different worktree/cwd, replaying the old cursor produces a stale or invalid resume. This change makes a thread start a fresh grok session whenever its workspace changes.

Changes

• ProviderCommandReactor: when the preferred provider is grok and the cwd changed, pass an explicit null resume cursor when restarting the session. null is a discard sentinel, distinct from undefined ("use the persisted cursor"). Adds shouldDiscardResumeCursorForCwdChange to the restart log.
• ProviderService: honor the null sentinel by dropping the cursor, and — independently — refuse to reuse a persisted cursor when an explicit cwd differs from the persisted one. Persisted-cwd lookup is hoisted so the cursor-reuse guard and effectiveCwd share it.

Tests

• Reactor: discards grok resume cursors when the thread workspace changes (asserts resumeCursor: null and the new cwd on the second startSession).
• Service: does not reuse a persisted resume cursor when an explicit cwd changes.

Note: the two files are intentionally in one PR — the reactor emits the null sentinel that the service-layer change is what interprets; split apart, the sentinel would be silently treated as "use persisted".

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