feat: add Git-backed project fixtures

[kitlangton]

Summary

• add declarative project.git.files setup for scripted Drive runs
• create a deterministic initial commit after project setup and before OpenCode launches
• isolate Git metadata and environment from the host, reserve .git, and include ignored fixture files in the baseline
• document the API and snapshot-enabled /undo workflow

export default defineScript({
  project: {
    git: {
      files: {
        "src/example.ts": "export const value = 1\n",
      },
    },
  },
  setup({ config }) {
    config.snapshots = true
  },
  async run({ ui, llm }) {
    // Simulated model responses can now call real file tools against a Git baseline.
  },
})

End-to-end proof

Tested against current OpenCode V2 using the real patch tool and /undo flow:

1. Drive created an isolated Git project containing a deeply nested fixture file.
2. The simulated model called OpenCode’s production patch tool.
3. OpenCode Snapshot.Service captured the change.
4. /undo populated revert.files and rendered …/navigation/deeply-nested/dialog-select-configuration.tsx with +1 -1.

A successful 3.75-second MP4 was recorded locally as recording-ddb072ae-5dcd-4da8-ae4c-e9fed1919dc1.mp4; no fixture or media artifacts are committed.

Verification

• bun run test: 71 passed
• bun run typecheck: passed
• bun run lint: 0 errors, 21 pre-existing warnings
• opencode-drive check /tmp/opencode-drive-git-project-proof.ts: passed
• real OpenCode V2 Drive recording: passed
• git diff --check: passed