feat(git): add GitLab merge request support alongside GitHub PRs

[tyulyukov]

Summary

• Introduced GitHostCli service contract that abstracts git host operations (PR/MR list, view, create, checkout)
• Implemented GitLabCli layer wrapping glab CLI alongside existing GitHubCli wrapping gh CLI
• Created RoutingGitHostCli to auto-detect provider from remote.origin.url hostname with fallback to git config and CLI auth probing
• Refactored GitManager to depend only on GitHostCli abstraction, removing GitHub-specific logic
• Updated web UI components to dynamically show "PR" or "MR" labels based on detected gitHostProvider
• Renamed GitHubCliError to GitHostCliError (GitHub-specific error) with deprecation aliases for backwards compatibility
• Updated GitCore to support configurable refspec prefix (refs/pull for GitHub, refs/merge-requests for GitLab)
• Gracefully defer fork-based MR workflows for GitLab with clear error messaging
• Updated all tests to work with the abstraction layer
• Updated AGENTS.md to document the Git Host Provider Abstraction pattern

Testing

• GitManager tests updated to work with GitHostCliShape abstraction
• PR listing, viewing, and creation flows tested with fake gh implementation
• Error handling for missing CLI and authentication failures tested
• Repository name parsing updated to handle both GitHub and GitLab URL formats
• bun fmt, bun lint, and bun typecheck passing