Epic: Support GitLab as a trigger source

[geoffjay]

Summary

Add gitlab_issues and gitlab_merge_requests as workflow trigger source types, following the existing patterns established by GitHub (gh CLI) and Linear (direct HTTP API). Also add assignee/assignees filtering to both GitLab and GitHub triggers.

Context

The orchestrator scheduler has a clean TaskSource trait + TriggerConfig tagged enum architecture supporting GitHub issues/PRs and Linear issues. GitLab needs equivalent support using the GitLab REST API v4, which allows both gitlab.com and self-hosted instances.

From #1178, the issue description notes that assignee was added to the issues trigger and assignees to the merge requests as an optional field, and the author has been meaning to add this to GitHub as well.

Architecture Decision: Direct HTTP API (not glab CLI)

Use the GitLab REST API v4 directly (like Linear) rather than the glab CLI (like GitHub) because:

• glab CLI is far less commonly installed than gh
• Direct API supports self-hosted GitLab via configurable base URL
• GitLab REST API is straightforward - no GraphQL needed
• Follows the Linear integration pattern already in the codebase

Configuration Pattern

Following LinearConfig:

• AGENTD_GITLAB_TOKEN env var (primary)
• [gitlab] section in ~/.config/agentd/config.toml (fallback)
• AGENTD_GITLAB_URL env var for self-hosted (default: https://gitlab.com)

YAML Template Format

# GitLab Issues
source:
  type: gitlab_issues
  owner: myorg
  repo: myrepo
  labels: [agent]
  assignee: user
  state: opened

# GitLab Merge Requests
source:
  type: gitlab_merge_requests
  owner: myorg
  repo: myrepo
  assignees: [user]
  state: opened

Key Files

• crates/orchestrator/src/scheduler/gitlab.rs (NEW)
• crates/orchestrator/src/scheduler/types.rs - TriggerConfig variants
• crates/orchestrator/src/scheduler/runner.rs - create_source() factory
• crates/orchestrator/src/scheduler/api.rs - validation
• crates/orchestrator/src/scheduler/template.rs - known variables
• crates/orchestrator/src/scheduler/mod.rs - module registration
• crates/cli/src/commands/apply.rs - SourceTemplate variants
• schemas/workflow.yml - schema definitions
• docs/public/trigger-reference.md - documentation

Child Issues

• Add GitLab config, trigger type variants, and module scaffolding
• Implement GitlabIssueSource task source
• Implement GitlabMergeRequestSource task source
• Add assignee/assignees filter fields to GitHub triggers
• Add API validation and credential checks for GitLab triggers
• Update workflow schema and documentation for GitLab triggers
• Add unit and integration tests for GitLab trigger sources

Related Issues

• Plan: Add Gitlab support for project tracking #456 - Plan: Add Gitlab support for project tracking (placeholder, superseded by this epic for trigger sources)
• Add additional task sources to orchestrator scheduler #10 - Add additional task sources to orchestrator scheduler (original meta-issue)
• Plan: Support Gitlab as a trigger source #1178 - Plan: Support Gitlab as a trigger source (planning issue)

Stack Base

Stack on: feature/autonomous-pipeline