feat: add .git-ai-ignore file support for custom ignore patterns

[svarlamov]

feat: add .git-ai-ignore file support for custom ignore patterns

Summary

Adds support for a .git-ai-ignore file at the repository root, allowing users to specify additional file patterns to exclude from git-ai tracking. The file uses .gitignore-style syntax (one glob pattern per line, # comments, blank lines skipped).

Patterns from .git-ai-ignore are unioned with all existing ignore sources in effective_ignore_patterns():

1. Built-in defaults (lockfiles, generated files, etc.)
2. .gitattributes linguist-generated patterns
3. .git-ai-ignore patterns ← new
4. Extra patterns (programmatic)
5. User CLI --ignore patterns

The implementation mirrors the existing .gitattributes loading: reads from workdir for normal repos, reads from HEAD for bare repos.

No callers were changed — all four call sites (checkpoint.rs, status.rs, post_commit.rs, git_ai_handlers.rs) go through effective_ignore_patterns(), so .git-ai-ignore is picked up automatically everywhere.

Review & Testing Checklist for Human

• Verify the simplified parsing is acceptable: The parser strips leading/trailing whitespace via trim() and skips #-prefixed lines, but does NOT support .gitignore negation (!pattern), escaped hashes (\#), or directory-only trailing slashes (dir/). Confirm this subset is sufficient for the intended use case.
• Confirm union ordering is correct: .git-ai-ignore patterns are inserted after .gitattributes linguist-generated and before extra/user patterns. Verify this doesn't cause unexpected precedence issues given that all patterns are additive (ignore-only, no un-ignore).
• Test manually: Create a repo with a .git-ai-ignore containing a pattern like docs/**, make changes to files matching that pattern, run git-ai checkpoint and git-ai status --json, and confirm those files are excluded from stats.

Notes

• The .git-ai-ignore file is read from the working directory (not the committed version) for non-bare repos, consistent with how .gitattributes is already handled. This means uncommitted .git-ai-ignore files take effect immediately.
• All 19 unit tests and 8 integration tests pass locally.
• Link to Devin run
• Requested by: @svarlamov

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✅ Devin Review: No Issues Found

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

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