Add API validation and credential checks for GitLab triggers

[geoffjay]

Summary

Add API-level validation and credential checking for GitLab trigger configurations in the orchestrator's workflow creation endpoint, following the pattern established for Linear triggers.

Context

When creating a workflow via POST /workflows, the orchestrator validates trigger configs (e.g., non-empty owner/repo for GitHub, at least one filter for Linear, valid cron expressions). GitLab triggers need similar validation plus a credential check so users get clear errors at workflow creation time rather than silent failures on the first poll.

Acceptance Criteria

• Add validation case for TriggerConfig::GitlabIssues in create_workflow() in crates/orchestrator/src/scheduler/api.rs:
  • owner and repo must be non-empty
  • state must be one of opened, closed, all (GitLab uses opened not open)
• Add validation case for TriggerConfig::GitlabMergeRequests:
  • owner and repo must be non-empty
  • state must be one of opened, closed, merged, all
• Add credential check (following Linear pattern at line 206):

  if matches!(req.trigger_config, TriggerConfig::GitlabIssues { .. } | TriggerConfig::GitlabMergeRequests { .. })
      && !crate::scheduler::gitlab::GitlabConfig::is_configured()
  {
      return Err(ApiError::InvalidInput(
          "GitLab token not configured. Set AGENTD_GITLAB_TOKEN or add 'token' to [gitlab] in config."
      ));
  }

• Add GitLab variant to WebhookSource enum in crates/orchestrator/src/scheduler/types.rs for future webhook support (optional, can be deferred)
• Ensure unknown/extra fields in GitLab trigger YAML produce clear deserialization errors

Key Files

• crates/orchestrator/src/scheduler/api.rs - validation logic
• crates/orchestrator/src/scheduler/types.rs - optional WebhookSource variant
• crates/orchestrator/src/scheduler/gitlab.rs - GitlabConfig::is_configured()

Blocked By

• The GitLab scaffolding issue (TriggerConfig variants and GitlabConfig must exist)

Stack Base

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