feat: GitLab & Bitbucket support in scan.sh

[Hrithik-Gavankar]

Summary

Extend the git scanner to work with GitLab and Bitbucket repositories, including support for their specific branch naming conventions, MR/PR metadata, and CI integration.

Current State

• scan.sh is git-native (works with any git repo locally)
• However, some features assume GitHub conventions (e.g., main/master as default branch)
• No integration with GitLab MRs or Bitbucket PRs for richer metadata

Proposed Enhancements

• Auto-detect default branch name (not just main/master)
• GitLab MR metadata via glab CLI (if available)
• Bitbucket PR metadata via Bitbucket CLI (if available)
• Support for GitLab CI pipeline status
• Support for Bitbucket Pipelines status
• Forge-agnostic PR/MR count in velocity metrics
• Remote URL parsing to identify forge type

Acceptance Criteria

• Scanner works identically for GitHub, GitLab, and Bitbucket repos
• Default branch auto-detected per repo (not hardcoded)
• Optional enrichment when forge CLIs are available
• No breaking changes to existing GitHub workflow