[Code Quality] Document antigravity engine in engines.md and update schema/code comments

[github-actions]

Description

The antigravity engine is fully implemented and used in production (e.g., .github/workflows/smoke-antigravity.md) but is completely omitted from:

• docs/src/content/docs/reference/engines.md — user-facing engine catalog
• pkg/parser/schemas/main_workflow_schema.json — engine enum descriptions
• pkg/workflow/engine_definition.go lines 13-20 and 184-185 — stale code comment listing built-ins

Additionally, frontmatter.md omits mention of advanced engine: subfields (driver, extensions), and frontmatter-full.md still publishes the deprecated dispatch_repository as a full example.

Suggested Changes

• Add antigravity to the engine list in docs/src/content/docs/reference/engines.md
• Update engine enum descriptions in pkg/parser/schemas/main_workflow_schema.json to include antigravity
• Fix stale code comments in pkg/workflow/engine_definition.go (lines 13-20, 184-185)
• Add a brief note in docs/src/content/docs/reference/frontmatter.md pointing to driver and extensions subfields
• Remove or deprecate the dispatch_repository example block in frontmatter-full.md

Files Affected

• docs/src/content/docs/reference/engines.md
• pkg/parser/schemas/main_workflow_schema.json
• pkg/workflow/engine_definition.go (lines 13-20, 184-185)
• docs/src/content/docs/reference/frontmatter.md
• docs/src/content/docs/reference/frontmatter-full.md

Success Criteria

• antigravity appears in the user-facing engine catalog
• Schema engine descriptions match runtime-supported engines
• Code comments in engine_definition.go list all current built-ins
• No deprecated dispatch_repository example in frontmatter-full.md

Source

Extracted from Schema Consistency Check #45389

Priority

Medium — Undocumented engines cause user confusion and doc/runtime drift

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• expires on Jul 14, 2026, 11:37 PM UTC-08:00