v0.0.17 - Remove git fetch, add discovery filtering and result aggregation

🚀 Major Performance Improvements

Remove git fetch operations

• 20+ minute improvement: Eliminated slow git fetch operations by using GitHub API
• Use gh api "repos/${REPO}/branches/{branch}" to get commit metadata directly
• Compare dates using epoch timestamps instead of git log --since

Discovery stage filtering

• 30-50% reduction in branches sent to process stage
• Pre-filter protected branches, default branches, and PR branches during discovery
• Reduces API calls and speeds up overall execution
• New inputs: default_branches, extra_protected_branch_regex, exclude_open_pr_branches

Result aggregation

• New summary job collects results from all parallel batches
• Displays formatted markdown summary in GitHub Step Summary
• Shows total deleted branches and errors across all batches
• Action outputs: deleted_branches, deleted_count, error_branches, error_count

Bug Fixes

• Fixed date parsing on Alpine Linux by adding GNU coreutils
• Added proper error handling for date conversion edge cases
• Fixed workflow output display by adding step IDs

📚 Documentation

• Updated MIGRATION.md with discovery filtering examples
• Added "Understanding Action Outputs" section with usage examples
• Updated parallel-delete-example.yml with complete workflow pattern

🔧 Technical Changes

• Added coreutils to Dockerfile for GNU date support
• Added filtering inputs to discovery/action.yml
• Enhanced error messages and logging throughout scripts