feat(health): config-aware health gating (telemetry + optional gate)

[mojoatomic]

Summary

• Add health telemetry and optional gating to ratchet.js (config-aware via readProjectConfig).
• Scoring: density per 1K executable LOC (overall/structural/semantic).
• Gate: gateOn (overall|structural|semantic), minOverall, intentOverrides (e.g., refactoring), bypass (env or commit token).
• Docs: HEALTH-GATING.md with config example and guidance; link from CONTEXT-AWARE-LINTING.md.

Behavior

• Disabled by default (health.enabled=false). Prints health scores (informational).
• When enabled and not bypassed, fails if current score < threshold.

Why

• Foundations for Health Gating Enforcement for Context-Aware Ratchet #203: enable calibrated, context-aware gating aligned with exLOC telemetry.

Tests (follow-up)

• Unit: computeHealth mapping.
• Integration: gate pass/fail and bypass token.

Notes

• Traditional ratchet behavior unchanged.
• Commit token bypass: set in .ai-coding-guide.json (e.g., [health-bypass]); env HEALTH_BYPASS=true also bypasses.

Refs

• Health Gating Enforcement for Context-Aware Ratchet #203 Health Gating Enforcement
• Ratchet 2.0: context-aware code health, critical vs minor policy, weighted/density metrics, heuristics (Phase 2 next) #198 Telemetry burn-in & promotion
• Executable line counting (exLOC) metrics: integrate into analyzer + context telemetry #204 exLOC metrics