Update workflow schema and documentation for GitLab triggers

[geoffjay]

Summary

Update the workflow YAML schema and public documentation to cover the new GitLab trigger types (gitlab_issues, gitlab_merge_requests), the new assignee/assignees fields on GitHub triggers, and the GitLab configuration requirements.

Context

The workflow schema (schemas/workflow.yml) and documentation (docs/public/) need to be updated to reflect the new trigger types. The schema is used for validation and IDE autocompletion; the docs are the primary reference for users.

Acceptance Criteria

Schema Updates (schemas/workflow.yml)

• Add source_gitlab_issues definition:
  • Required: type, owner, repo
  • Optional: labels (array), state (enum: opened/closed/all, default: opened), assignee (string)
  • Template variables documented: {{title}}, {{body}}, {{url}}, {{labels}}, {{assignee}}, {{source_id}}, {{gitlab_project_id}}, {{gitlab_iid}}
• Add source_gitlab_merge_requests definition:
  • Required: type, owner, repo
  • Optional: labels (array), state (enum: opened/closed/merged/all, default: opened), assignees (array of strings)
  • Template variables documented: includes {{source_branch}}, {{target_branch}}, {{merge_status}}, {{draft}}
• Add both refs to source_template.oneOf list
• Add assignee property to source_github_issues definition
• Add assignees property to source_github_pull_requests definition
• Add examples for GitLab triggers

Documentation Updates

• Update docs/public/trigger-reference.md:
  • Add gitlab_issues and gitlab_merge_requests rows to comparison table
  • Add "When to Use" section for GitLab triggers
  • Add configuration section explaining AGENTD_GITLAB_TOKEN and AGENTD_GITLAB_URL
  • Document GitLab state values (opened vs GitHub's open)
• Update docs/public/templates.md:
  • Add GitLab trigger type to template variable tables
  • Document GitLab-specific variables
  • Add example workflow YAML for GitLab
• Document the new assignee/assignees fields on GitHub triggers in both files

Key Files

• schemas/workflow.yml - YAML schema definitions
• schemas/definitions.yml - shared definitions (if webhook_source needs updating)
• docs/public/trigger-reference.md - trigger comparison and configuration
• docs/public/templates.md - template syntax and variable reference

Blocked By

• The GitLab scaffolding issue (type names and field definitions must be finalized)

Stack Base

Branch off: feature/autonomous-pipeline
Blocked by: #1180