feat(tracker-gitlab): add GitLab tracker plugin

[mag1art]

This PR adds a new tracker plugin for GitLab Issues:

• packages/plugins/tracker-gitlab: production-ready plugin implementing the Tracker interface
  • getIssue, isCompleted, issueUrl, branchName, generatePrompt
  • createIssue, listIssues, updateIssue (best-effort mapping)
  • webhook validation helper
• Unit tests (vitest/nock) and README with configuration instructions
• Plugin registered in core plugin registry (packages/core/src/plugin-registry.ts)

Notes:

• Requires GITLAB_TOKEN (Personal Access Token with api scope).
• Self-hosted GitLab supported via baseUrl config.

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