Add undocumented flag to BaseEngine to stop antigravity drift-detection churn

[Copilot]

The antigravity engine is registered and functional in production but intentionally absent from engines.md and main_workflow_schema.json. Because nothing in code recorded this exclusion, drift-detection tools kept re-flagging it as missing documentation — resulting in 10+ duplicate not_planned issues and three rejected fix PRs.

Changes

• BaseEngine — new undocumented bool field (default false; all existing engines unaffected) with IsUndocumented() bool accessor. Drift tools check this before comparing registered engines against docs/schema.

• EngineRegistry.GetDocumentedEngines() []string — returns only engines where IsUndocumented() == false, sorted. Drift checks should call this instead of GetSupportedEngines() to exclude intentionally hidden engines.

• NewAntigravityEngine() — sets undocumented: true with an explanatory comment, encoding the exclusion as durable, machine-readable intent.

// Before: no way to distinguish "not yet documented" from "intentionally undocumented"
registry.GetSupportedEngines() // → ["antigravity", "claude", "codex", ...]

// After: drift checks use the documented subset
registry.GetDocumentedEngines() // → ["claude", "codex", "copilot", "gemini", "opencode", "pi"]
registry.IsValidEngine("antigravity") // → true (still usable)
NewAntigravityEngine().IsUndocumented() // → true

• Fixes [doc-healer] DDUw improvement - stop re-processing rejected antigravity engine-catalog direction #47159

[github-actions]

🤖 PR Triage

Field	Value
Category	refactor
Risk	medium
Score	50/100
Breakdown	impact: 22 · urgency: 15 · quality: 13
Action	batch_review
Batch	go-code (with #47088, #47108)

Rationale: Behavioral change to engine registry API — GetDocumentedEngines() is a new contract that drift tools must adopt. Small footprint (5 files, +87/-1), CI ✅, draft. Good quality but needs human validation of the registry contract before undraft.

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