Expose stack collapse in merge train runner

[cbusillo]

Summary

• add workflow-dispatch inputs for merge-train stack-collapse execute/admit phases
• route those inputs through the deployed Launchplane service using the same OIDC trust boundary as existing runner phases
• keep batch, stack-collapse, and Level 1 modes mutually exclusive per dispatch

Validation

• Ruby YAML parse of .github/workflows/merge-train-runner.yml
• git diff --check
• actionlint will run in PR Security workflow on the self-hosted runner because Docker is not available locally