Ignore empty nested .git directories

[svarlamov]

Fixes #1415

Summary

• require a HEAD marker before treating a directory-form .git entry as a repository
• reuse the validation for no-exec discovery and nested repository boundaries without adding Git subprocesses or extra normal-path filesystem lookups
• update synthetic daemon test fixtures to model valid Git directories

Test coverage

• add a TestRepo regression covering pre/post AI checkpoints beneath an empty nested .git directory
• task test
• task lint
• task fmt

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[svarlamov]

• Ignore empty nested .git directories #1910 👈 (View in Graphite)
• main

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