feat: quality gate — threshold-based pass/fail for CI (#205)

[ajianaz]

Summary

Implements configurable quality gate for CI enforcement. Closes #205 (partial).

New

• src/engine/quality_gate.rs — gate evaluation logic (558 lines)
• Config via .cora.yaml under quality_gate section
• 12 unit tests covering all scenarios

Config Example

quality_gate:
  enabled: true
  thresholds:
    max_critical: 0
    max_security: 0
  categories:
    performance:
      action: warn
      max_findings: 5

Output

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
  QUALITY GATE RESULT
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
  Status:   ❌ FAILED
  Findings: 2 critical, 1 major, 4 minor, 0 info

  Threshold Checks:
  ✅ max_critical          → 2 found   ❌ EXCEEDED
  ✅ max_security          → 0 found   ✅ OK
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Test Results

• 398 tests passed (12 new quality_gate tests)
• Release build OK