Add undocumented marker to BaseEngine; record antigravity exclusion decision in code

[Copilot]

The antigravity engine is registered and used in production but intentionally absent from engines.md — a docs PR (#42294) adding it was closed unmerged by a maintainer. Because nothing in code recorded this intent, drift detectors (DeepReport, schema-consistency, doc-healer) kept re-filing the same reconciliation issues each cycle.

Changes

• Engine interface + BaseEngine — adds IsUndocumented() bool method (defaults false). New undocumented bool field on BaseEngine carries the exclusion intent.
• EngineRegistry.GetDocumentedEngines() — new method returning sorted engine IDs where IsUndocumented() == false. Drift detectors should call this instead of GetSupportedEngines() when comparing against engines.md/schema.
• NewAntigravityEngine() — sets undocumented: true with a comment referencing the closed PR decision.
• Tests — TestSpec_Engine_DocumentedEnginesRegistered no longer lists antigravity; new TestSpec_Engine_AntigravityIsUndocumented asserts the engine remains runtime-registered while GetDocumentedEngines() excludes it.

// Before: every registered engine appeared as a docs drift candidate
registry.GetSupportedEngines() // ["antigravity", "claude", "codex", ...]

// After: drift detectors use this instead
registry.GetDocumentedEngines() // ["claude", "codex", "copilot", "crush", "gemini", "opencode", "pi"]

// antigravity still works at runtime
registry.GetEngine("antigravity") // ok — undocumented ≠ unregistered

No engines.md or schema change — the exclusion is now the enforced expectation.