[Code Quality] Add antigravity engine to docs and schema built-in engine catalog

[github-actions]

Description

The antigravity engine is supported by the runtime (pkg/workflow/engine_definition.go:18,285) and used in real workflows (.github/workflows/smoke-antigravity.md), but it is omitted from:

1. Engine reference docs — docs/src/content/docs/reference/engines.md:16-21 lists only Copilot, Claude, Codex, Gemini, OpenCode, and Pi.
2. Schema examples/descriptions — pkg/parser/schemas/main_workflow_schema.json engine ID examples/descriptions mention codex, claude, copilot, gemini, opencode, and pi only.

This makes valid workflows appear undocumented and steers users away from a supported engine.

Suggested Changes

1. Add antigravity to the built-in engine list in docs/src/content/docs/reference/engines.md (alongside the other engines, with a brief description).
2. Add antigravity to schema descriptions/examples for engine.id in pkg/parser/schemas/main_workflow_schema.json wherever other built-in engine IDs are enumerated.
3. Consider adding a short engine.extensions section to the concise docs/src/content/docs/reference/frontmatter.md (currently only documents engine.driver; engine.extensions exists in schema and is implemented in pkg/workflow/pi_engine.go:211,240).

Files Affected

• docs/src/content/docs/reference/engines.md
• pkg/parser/schemas/main_workflow_schema.json
• docs/src/content/docs/reference/frontmatter.md (for engine.extensions)

Success Criteria

• antigravity appears in the engines reference doc with a description.
• Schema examples for engine.id include antigravity.
• Users can discover antigravity as a supported engine from docs without reading source code.
• (Bonus) engine.extensions is documented in frontmatter.md.

Source

Extracted from Schema Consistency Check - 2026-07-20 #46731

Priority

Medium — documentation gap for a supported engine; users may avoid or be confused by a valid engine.

🔍 Task mining by Discussion Task Miner - Code Quality Improvement Agent · 39.8 AIC · ⌖ 5.41 AIC · ⊞ 7K · ◷

• expires on Jul 21, 2026, 11:21 AM UTC-08:00

[github-actions]

This issue was automatically closed because it expired on 2026-07-21T19:21:50.081Z.

Closed by Workflow