feat: add ignore patterns support for stats command

[txf0096]

Fixes #461

Summary

This PR implements ignore patterns support for the stats command, allowing users to exclude auto-generated files (lock files, minified code, generated code) from AI/human statistics.

Implementation

Added three configuration levels (all additive):

1. CLI flag

git-ai stats --ignore "*.lock" "*.min.js" "dist/**"
git-ai stats HEAD --ignore "*.generated.*"

2. Project-level config

Create .git-ai-ignore in repository root (gitignore syntax):

# Lock files
*.lock
go.sum

# Build artifacts  
*.min.js
*.bundle.js

# Generated code
*.generated.*
*_pb.go

3. Global config

In ~/.git-ai/config.json:

{
  "stats_ignore_patterns": [
    "*.lock",
    "*.min.js",
    "*.generated.*"
  ]
}

Changes

• Add src/authorship/ignore_patterns.rs module for loading patterns from files
• Add stats_ignore_patterns field to Config struct
• Merge patterns in handle_stats() before calling stats/range functions
• Only affects stats command (not checkpoint, blame, diff)
• Uses glob pattern matching (like .gitignore)
• Skips entire files from statistics (both AI and human lines)

Benefits

✅ More accurate AI/human code ratio
✅ Exclude noise from lock files and generated code
✅ Team consistency via committed .git-ai-ignore
✅ Flexible: global defaults + per-project overrides

Example

Before:

$ git-ai stats HEAD
you  ████████░░░░░░░░░░░░ ai
     65%              35%
# Includes 5000 lines from package-lock.json

After:

$ git-ai stats HEAD
you  ████████████████░░░░ ai
     80%              20%  
# package-lock.json excluded via .git-ai-ignore

Testing

• Compiled successfully with cargo build
• Pattern loading logic tested with multiple sources
• Backward compatible (defaults to no patterns if not configured)

---

[devin-ai-integration]

✅ Devin Review: No Issues Found

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

View in Devin Review to see 4 additional findings.