fix: prevent Antigravity session resume wedge and use atomic git worktree add -b #28354

Summary

1. Antigravity ACP sessions can wedge after an interrupted turn (Failed to launch two-stage session)

   • Interrupted antigravity-acp turns can leave trailing steps rows in <appDataDir>/brain/<conversationId>/conversation.db with status = 'WAITING', causing subsequent session activations to fail or wedge.
   • AcpCoreAdapter.activateSession() always calls session/load before session/resume, replaying the full history stream (which can exceed the 30s RPC timeout on long threads) even when unstable_resumeSession is supported.
   • Timed-out session/cancel or failed activateSession calls can leave the cached ACP child process in a bad state, and retireRuntime fired options.onTermination twice.

2. GitVcsDriver.createWorktree creates the branch in a separate step before `git worktree add

   • createWorktree runs createBranchAt (git branch --no-track -- <newBranch> <startPoint>) in the parent repository before running git worktree add -- <path> <newBranch>.
   • If git worktree add fails (e.g., target path collision or checkout failure), the newly created local branch is left orphaned in the repository.

Proposed Fix

• AntigravitySessionFiles.ts: Sanitize conversation.db before starting/resuming by updating any stuck status = 'WAITING' rows in steps to 'CANCELED'.
• AcpCoreAdapter.ts / AntigravityAdapterV2.ts: Add preferResumeSession: true so Antigravity attempts session/resume before session/load when supported.
• AcpCoreAdapter.ts / AntigravityAdapterV2.ts: Mark runtimeRestartRequired = true when activateSession fails, runtime.cancel fails, or the 10s interrupt wait times out, and ensure options.onTermination fires once from the process exit handler.
• GitVcsDriverCore.ts: Create the worktree and branch atomically in one step via git worktree add --no-track -b <newBranch> -- <worktreePath> <startPoint>, then configure upstream tracking only when startPoint resolves to refs/remotes/*.