[Code Quality] Add antigravity engine to docs/reference/engines.md and main_workflow_schema.json

[github-actions]

Description

The antigravity engine is supported at runtime (listed in pkg/workflow/engine_definition.go built-in catalog and used in .github/workflows/smoke-antigravity.md) but is absent from the user-facing engine reference documentation and the workflow schema's engine examples.

Suggested Changes

• Add antigravity to the engine list in docs/src/content/docs/reference/engines.md
• Add antigravity to the built-in engine ID examples/enum in pkg/parser/schemas/main_workflow_schema.json
• Verify schema examples near runtime.id include all current built-in engines

Files Affected

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

Success Criteria

• antigravity appears in the engine reference docs alongside codex, claude, copilot, gemini, opencode, pi
• Schema examples reflect the full set of supported built-in engine IDs
• No behavioral changes; all existing tests pass

Source

Extracted from [Schema Consistency] Schema Consistency Check 2026-07-22 #47210

Priority

Medium — users reading docs cannot discover a supported engine; impacts discoverability and trust in documentation accuracy

🔍 Task mining by Discussion Task Miner - Code Quality Improvement Agent · sonnet46 38.9 AIC · ⌖ 8.11 AIC · ⊞ 7.1K · ◷

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

[github-actions]

This issue was automatically closed because it expired on 2026-07-24T01:19:37.312Z.

Closed by Workflow