Add GitLab support

[cyberstorm-nisto]

Summary

Extend didgit.dev to support GitLab identity attestations.

Implementation

Required

• GitLab OAuth integration
• GitLab Snippet proof mechanism (instead of Gist)
• GitLab Events API for contribution tracking
• GitLab commit verification

Technical Details

• Domain: gitlab.com (or custom for self-hosted)
• Proof: Public GitLab Snippet containing signed binding
• API: GitLab REST API v4

See EXTENDING.md for implementation guide.

Acceptance Criteria

• User can bind gitlab.com:username to wallet
• User can create proof snippet via GitLab OAuth
• Contributions from GitLab repos can be attested
• Self-hosted GitLab instances supported via custom domain