fix: encode antigravity undocumented intent to break DDUw false-positive loop

[Copilot]

antigravity is production-registered but intentionally excluded from user-facing docs. Because that exclusion was purely social (maintainer PR closures), drift detectors had no machine-readable signal and kept re-filing the same engines.md gap — 13 duplicate issues in 7 days.

Go: pkg/workflow/

• Engine interface — new IsUserFacingDocumented() bool method; default implementation on BaseEngine returns !undocumented (default true for all engines)
• BaseEngine — new undocumented bool field; when true, excludes the engine from drift-detector reconciliation without affecting runtime availability
• EngineRegistry — new GetDocumentedEngines() []string returns only engines where IsUserFacingDocumented() == true; this is the set detectors must reconcile against instead of GetSupportedEngines()
• NewAntigravityEngine() — sets undocumented: true with a comment referencing the maintainer decision history

// pkg/workflow/antigravity_engine.go
BaseEngine{
    id:           "antigravity",
    undocumented: true, // intentionally excluded from engines.md — see gh-aw#47875
    ...
}

Tests: pkg/workflow/spec_test.go

• TestSpec_Engine_GetDocumentedEngines — asserts antigravity absent from GetDocumentedEngines(), present in GetSupportedEngines()
• TestSpec_Engine_IsUserFacingDocumented — asserts correct value per engine

Workflow: .github/workflows/daily-doc-updater.md

• Step 4 — new documented-engines-only rule: before flagging a missing engine row, check grep -r "undocumented:" pkg/workflow/; skip engines with undocumented: true
• Step 1c — two new guards:
  1. Intentionally-undocumented engine early-exit: if a closed issue concerns an undocumented: true engine, skip unconditionally regardless of closure state
  2. Rejection-awareness: if ≥2 PRs with the same fix direction were closed unmerged, treat the direction as rejected and do not re-attempt without a fresh maintainer decision

• Fixes [doc-healer] DDUw improvement: antigravity engine-doc drift re-escalation (13 re-files in 7 days, prior escalation #46688 expire [Content truncated due to length] #47875

[github-actions]

Triage | Category: bug | Risk: medium | Score: 51/100 (impact:25, urgency:15, quality:11)

Recommended action: fast_track — Antigravity false-positive loop fix; modifies workflow engine + lock files. Still draft; needs undraft + CI before merge.

Score breakdown: impact=25 (behavior fix, engine-critical path), urgency=15 (recent, no CI yet), quality=11 (no CI, good description)

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[github-actions]

🤖 Great work on this fix! This PR elegantly encodes the antigravity engine's intentional exclusion from user-facing docs into machine-readable form, breaking the 13-issue-in-7-days false-positive loop.

The changes are focused and well-tested:

• ✅ New IsUserFacingDocumented() interface method lets drift detectors distinguish between supported and documented engines
• ✅ GetDocumentedEngines() on the registry provides the authoritative set for documentation reconciliation
• ✅ Tests cover both the presence in the supported set and absence in the documented set
• ✅ Workflow guards prevent re-filing and add rejection-awareness for already-closed fix directions

This PR is ready for review. 🎯

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