Sync built-in engine catalog references for antigravity and pi

[Copilot]

The built-in engine.id catalog had drifted across schema, generated docs, and code comments. antigravity was already supported at runtime but was missing from schema/docs, and some code/test surfaces still reflected an older built-in set.

• Schema + generated docs

  • Updated $defs.engine_config descriptions in main_workflow_schema.json to list the current built-ins:
    • antigravity, claude, codex, copilot, crush, gemini, opencode, pi
  • Synced the schema-derived reference docs and editor autocomplete metadata so the documented catalog matches the accepted runtime catalog.

• Code comments

  • Corrected stale built-in engine lists in pkg/workflow/engine_definition.go so the catalog comment matches the actual embedded engine definitions.

• Regression coverage

  • Updated engine registry/catalog tests that still assumed the older built-in set.
  • Extended catalog resolution coverage for antigravity, including its current definition metadata.

engine:
  id: antigravity

This PR aligns the three user-facing/static surfaces with the built-in engines already present in the runtime catalog and in-repo workflows.