[Code Quality] Add Antigravity engine to engines reference documentation

[github-actions]

Description

The antigravity engine is implemented, registered in the runtime catalog, and used in production workflows (e.g., .github/workflows/smoke-antigravity.md), but it is absent from the engines reference documentation at docs/src/content/docs/reference/engines.md. Users consulting the reference will incorrectly conclude engine: antigravity is unsupported.

Suggested Changes

• Add an Antigravity row to the "Available Coding Agents" table in docs/src/content/docs/reference/engines.md
• Include name, description, and supported configuration keys
• Add a regression check that compares runtime engine catalog entries (pkg/workflow/engine_definition.go) against the engines reference table

Files Affected

• docs/src/content/docs/reference/engines.md (add Antigravity entry)
• pkg/workflow/engine_definition.go (line 18, reference for catalog)
• .github/workflows/smoke-antigravity.md (evidence of real usage)

Success Criteria

• Antigravity appears in the engines reference table
• Documentation accurately reflects all runtime-supported engines
• CI or a test validates doc/runtime catalog parity

Source

Extracted from Schema Consistency Check - 2026-07-19 #46539

Priority

Medium — users are being misled by incomplete documentation for a supported engine

🔍 Task mining by Discussion Task Miner - Code Quality Improvement Agent · 83.8 AIC · ⌖ 5.28 AIC · ⊞ 7K · ◷

• expires on Jul 19, 2026, 11:45 PM UTC-08:00

[github-actions]

This issue was automatically closed because it expired on 2026-07-20T07:45:28.905Z.

Closed by Workflow

[github-actions]

This issue was automatically closed because it expired on 2026-07-20T07:45:28.905Z.

Closed by Workflow

[github-actions]

This issue was automatically closed because it expired on 2026-07-20T07:45:28.905Z.

Closed by Workflow

[github-actions]

This issue was automatically closed because it expired on 2026-07-20T07:45:28.905Z.

Closed by Workflow