refactor(opencode): speed up file service test fixtures

[esadomer]

Issue for this PR

N/A (refactor-only test-speed cleanup)

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Speeds up test/file/index.test.ts by removing git: true from file service cases that do not assert repository state. The .git exclusion case now creates a plain .git directory, so the coverage remains without paying for full repo setup.

How did you verify your code works?

• bun test --timeout 30000 test/file/index.test.ts from packages/opencode
• Before: 33.002s median from 31.354, 33.002, 34.678
• After: 15.981s median from 16.757, 15.981, 15.821
• Net: 17.021s faster, 51.6% lower wall time, 2.06x faster
• bun typecheck currently fails on unrelated upstream errors in src/cli/cmd/run/stream.transport.ts, src/cli/cmd/tui/component/dialog-workspace-create.tsx, test/cli/run/stream.transport.test.ts, and packages/enterprise/src/custom-elements.d.ts.

Screenshots / recordings

N/A

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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