fix(test): remove failing git-based fixture cleanup

[jdalton]

Fixes test failure in cmd-fix.integration.test.mts where git-based fixture cleanup was failing when no changes existed to revert.

Replaced git checkout/clean approach with tmpdir-based isolation helpers following the pattern used in optimize tests.

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Note

Replaces git-based fixture cleanup in cmd-fix.integration.test.mts with tmpdir-based utilities and removes related lifecycle hooks.

• Tests (src/commands/fix/cmd-fix.integration.test.mts)
  • Replace git-based fixture reset with tmpdir-based helpers: copyDirectory and createTempFixture.
  • Remove beforeAll/afterEach/afterAll cleanup hooks tied to git.
  • Keep existing flag, help, output, and error-handling test cases unchanged functionally.

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