test(opencode): migrate file fixtures to app filesystem

[kitlangton]

Summary

• migrate file test fixture writes/removals from direct fs promises to AppFileSystem.use helpers
• keep git command wrappers and direct AppFileSystem service behavior assertions intact
• preserve all existing file/index test names

Verification

• bunx prettier --write packages/opencode/test/file/index.test.ts
• bunx oxlint packages/opencode/test/file/index.test.ts
• bun typecheck (packages/opencode)
• bun test --timeout 5000 test/file/index.test.ts (packages/opencode)