feat: v2.2.0 pr cleanup and management

[clawdeeo]

Implements ghitgud v2.2.0 PR workflow commands.

Part 1: Cleanup

• Delete merged branches locally and remotely
• Fast-forward base branch (main/master)
• --dry-run to preview changes
• --force to skip ahead-of-base safety check
• Squash/rebase merges are detected and skipped safely

Part 2: Push

• ghitgud pr push to push local changes back to contributor's fork

Part 3: Stack

• ghitgud pr stack init --base to initialize stacked PR chains
• ghitgud pr stack add --depends-on to build dependency chains
• ghitgud pr stack status to view chain state
• ghitgud pr stack sync to rebase all stacked branches

Part 4: Next

• ghitgud pr next to checkout the next PR in dependency chain
• ghitgud pr next --reverse to checkout the previous PR

Part 5: Tests

• Unit tests for pr service
• Unit tests for stack service
• Unit tests for core/git operations

Part 6: Documentation

• README.md updated with PR command documentation
• CHANGELOG.md updated with v2.2.0 release notes