feat(runtime): Grok Build multi-turn session resume

Parent

https://github.com/nexu-io/open-design/issues/5946

What to build

Add multi-turn Grok Build sessions in Studio using Grok's native session resume, so follow-up design directions keep context without re-sending the full web transcript every turn.

End-to-end path:

1. Depends on streaming runtime (#5948): each completed Grok turn emits an end event with sessionId.
2. Daemon captures that id (capture-style resume) and stores it for the (conversation, agent) pair using the existing agent session resume machinery and invalidation guards (model change, cwd change, conversation advanced).
3. On a later turn with a valid stored session, spawn Grok with --resume <sessionId> and only the latest user turn as the composed user payload (no double-packed transcript).
4. Do not use Grok's -c / continue-most-recent-in-cwd (ambiguous across projects).
5. If resume fails (expired/missing session), clear the stale handle and reseed with a full transcript path consistent with other capture-style adapters—without flashing a misleading hard failure when transparent recovery is intended.

Product decisions (from parent PRD):

• resumesSessionViaCli + capture-from-stream for grok-build.
• Resume is a vertical slice on top of streaming; do not re-litigate stream parsing except as needed to emit/persist sessionId.
• Skills still stage per OD's shared skill composition rules; first turn establishes context in Grok's session memory.

Acceptance criteria

• After a successful Grok turn, a session id from end is stored for that conversation/agent
• Second turn in the same Studio conversation passes --resume <sessionId> in the spawned argv (visible in daemon log)
• Follow-up brief such as "change the hero copy" continues prior design context without pasting the full prior transcript into the prompt
• Resume is skipped/invalidated when model or project cwd changes (or other existing resume guards fire); a fresh session is used
• Resume failure clears stale handle and recovers via reseed rather than leaving the conversation permanently stuck
• Unit or focused tests cover buildArgs/resume wiring and/or session id capture from a fixture end event
• Agent-adapter docs mention Grok resume (--resume, capture-style, no -c)
• Manual E2E: two-turn design session described in the PR body

Blocked by

• https://github.com/nexu-io/open-design/issues/5948