test(health): unit + integration coverage for health gating (#214)

[mojoatomic]

Adds tests for #214 (and supports #203):

Unit

• lib/utils/health.computeHealth: 100s at zero density, 0 at perK=10, rounding at partial densities

Integration

• Threshold semantics: pass when score == threshold (>=)
• gateOn selection: structural pass vs semantic fail on same inputs
• intentOverrides: fail at default floor, pass with --intent=refactoring
• bypass: HEALTH_BYPASS env and commitToken in last commit message

Refactor

• Extracted computeHealth to lib/utils/health.js for testability (ratchet requires it via __dirname)

Notes

• Keeps deltas=0 in gating-focused tests to isolate health gate behavior from classic ratchet.

Closes #214.