docs: update GitLab integration to use GitLab Duo provider

[vglafirov]

Replace Anthropic provider with native GitLab Duo integration:

• Configure GitLab Duo provider with CI_JOB_TOKEN authentication
• Add @gitlab/opencode-gitlab-plugin for GitLab API tools
• Remove all custom CI/CD variables (ANTHROPIC_API_KEY, GITLAB_HOST, GITLAB_TOKEN_OPENCODE)
• Use GitLab predefined variables (CI_JOB_TOKEN, CI_SERVER_HOST, CI_PROJECT_PATH)
• Remove glab CLI dependency - plugin provides direct API access
• Add 'Using GitLab Tools' section with practical examples
• Update documentation link to opencode-gitlab-plugin
• Simplify setup - zero custom configuration required

The configuration now uses only GitLab's built-in CI/CD variables and provides native integration with GitLab Duo for AI-powered assistance.