feat(core): add git types and extractor infrastructure (#91)

[prosdev]

Summary

Adds the foundational types and infrastructure for Git history integration as part of Epic #90 (Intelligent Git History v0.4.0).

Changes

New Types (packages/core/src/git/types.ts)

• GitCommit - Complete commit information with author, committer, files, stats, refs
• GitFileChange - File change details (path, status, additions, deletions)
• GitPerson - Author/committer information
• GitRefs - Extracted issue/PR references from commit messages
• GitBlame / GitBlameLine - Line-level blame information
• ContributorStats - Contributor statistics (for future use)
• GitRepositoryInfo - Repository metadata

GitExtractor Interface (packages/core/src/git/extractor.ts)

• Abstract GitExtractor interface for future pluggability (GitHub API, etc.)
• LocalGitExtractor implementation that shells out to git commands
• Supports:
  • getCommits(options) - Fetch commits with filtering (limit, since, until, author, path)
  • getCommit(hash) - Get single commit by hash
  • getBlame(file, options) - Line-level blame information
  • getRepositoryInfo() - Repository metadata
• Automatic extraction of issue (feat(core): implement incremental indexing #123) and PR (PR #456) references from commit messages
• Parses --numstat output for accurate file change statistics

Tests

• 19 comprehensive unit tests covering:
  • Commit fetching and parsing
  • Author/committer information
  • File change tracking
  • Issue/PR reference extraction
  • Empty repository handling
  • Blame functionality
  • Repository info detection

Design Decisions

1. Extensible Interface: GitExtractor is an interface to allow future swapping of local git for GitHub API
2. Rich Metadata: Types include fields for future features (cross-repo, contributor stats)
3. Robust Parsing: Uses custom markers for reliable multi-line commit message parsing
4. Reference Extraction: Automatically identifies issue/PR refs for future linking

Closes

Closes #91

Part of

Epic: Intelligent Git History (v0.4.0) #90