[Code Quality] Add antigravity engine to docs and JSON schema engine catalog

[github-actions]

Description

The antigravity engine is fully supported at runtime (listed in pkg/workflow/engine_definition.go) but is missing from two user-facing surfaces:

1. docs/src/content/docs/reference/engines.md — no mention in the available engine table or built-in engine descriptions.
2. pkg/parser/schemas/main_workflow_schema.json — absent from inline engine runtime examples.

This means users can successfully use a supported engine that docs imply does not exist.

Suggested Changes

• Add antigravity to the available engine table in docs/src/content/docs/reference/engines.md alongside codex, claude, copilot, gemini, opencode, pi.
• Add antigravity to the built-in engine descriptions / examples in pkg/parser/schemas/main_workflow_schema.json (near runtime.id examples).
• Optionally add a brief description of the antigravity engine purpose in docs.

Files Affected

• docs/src/content/docs/reference/engines.md
• pkg/parser/schemas/main_workflow_schema.json

Success Criteria

• antigravity listed in the engine reference docs
• Schema examples include antigravity
• Schema validation no longer under-represents supported engine IDs

Source

Extracted from Schema Consistency Check discussion 47210 (Critical Issue 1 and 2)

Priority

Medium — documentation gap; engine is already functional, estimated effort < 1h

🔍 Task mining by Discussion Task Miner - Code Quality Improvement Agent · sonnet46 76.4 AIC · ⌖ 5.54 AIC · ⊞ 7.1K · ◷

• expires on Jul 23, 2026, 5:19 AM UTC-08:00

[github-actions]

This issue was automatically closed because it expired on 2026-07-23T13:19:34.754Z.

Closed by Workflow