[Code Quality] Add engine antigravity to schema enum and docs reference

[github-actions]

Description

The antigravity engine is supported in the runtime and used in production workflows (.github/workflows/smoke-antigravity.md), but is missing from:

1. Schema: pkg/parser/schemas/main_workflow_schema.json still lists only codex, claude, copilot, gemini, opencode, pi as valid engine.id values (lines 12532-12533)
2. Docs: docs/src/content/docs/reference/engines.md omits antigravity from the available engines table

This causes users to incorrectly think antigravity is unsupported or invalid.

Suggested Changes

1. Update pkg/parser/schemas/main_workflow_schema.json lines 12532-12533 to include antigravity in the engine.id description and/or enum
2. Add antigravity row to the engines table in docs/src/content/docs/reference/engines.md
3. Optionally add a schema enum (or clear docs note) that arbitrary IDs are allowed for custom runtimes

Files Affected

• pkg/parser/schemas/main_workflow_schema.json (lines 12530-12535)
• docs/src/content/docs/reference/engines.md

Success Criteria

• Schema description/examples include antigravity
• Engines docs table includes antigravity
• make recompile passes
• No regression in schema validation tests

Source

Extracted from [Schema Consistency] Schema Consistency Check - 2026-07-25

Priority

Medium — Closes documentation/schema gap for a production-supported engine.

🔍 Task mining by Discussion Task Miner - Code Quality Improvement Agent · sonnet46 · 47 AIC · ⌖ 5.49 AIC · ⊞ 7.2K · ◷

• expires on Jul 26, 2026, 11:12 AM UTC-08:00

[github-actions]

This issue was automatically closed because it expired on 2026-07-26T19:12:30.684Z.

Closed by Workflow