Add Gerrit forge adapter

[abhinavgautam01]

Summary

Adds initial Gerrit support as a forge adapter.

This maps Gerrit projects, branches, tags, files, and changes into the existing forge interfaces where they fit cleanly. Unsupported Gerrit concepts explicitly return ErrNotSupported.

Closes #12.

What works

• Adds gerrit forge type and detection via /config/server/version
• Adds Gerrit REST client with XSSI prefix stripping
• Supports repositories/projects
• Supports branch list/create/delete
• Supports tag listing
• Supports file content retrieval
• Maps Gerrit changes to pull requests for get/list/diff/comment/submit/abandon/restore
• Supports reviewer request/removal and review submission
• Adds explicit unsupported-service stubs for issues, labels, milestones, releases, CI, secrets, deploy keys, notifications, collaborators and commit statuses
• Updates CLI help, config docs, README and token lookup

Tests

• go test ./...
• git diff --check