feat(fusion): v1 completion — CLI subcommand, evaluator model, confidence-aware difficulty gate - #392

…nce, confidence-aware difficulty gate

1. sin-code fusion CLI subcommand (issue #290):
   - fusion status: enabled/disabled, oracle mode, difficulty gate, provider count
   - fusion config: full config keys + env var overrides (SIN_EVALUATOR_MODEL, etc.)
   - fusion providers: lists Fireworks pool (model, base URL, max tokens)
   - Read-only, no side effects. Registered in main.go

2. Oracle judge prefers SIN_EVALUATOR_MODEL (anti-bias):
   - WireFusion now uses firstNonEmpty(SIN_EVALUATOR_MODEL, cfg.Model) for judge
   - SIN_EVALUATOR_BASE_URL creates a separate client for the judge
   - Mirrors the stop-gate pattern at line 334

3. Confidence-aware difficulty gate fully wired:
   - TournamentRunner interface extended with ShouldRunWithConfidence
   - Loop passes verifyFailCount as attemptCount
   - fusionAdapter uses ShouldTournamentWithConfidence when signals available
   - classifyVerifyError maps verify.Result.Report to ErrorType
   - Falls back to text-only ShouldTournament when confidence=0 && attemptCount=0
   - All mock implementations updated (wiring_test, e2e_test)

Tests: fusion ✅, loopbuilder ✅, orchestrator ✅, agentloop ✅ (all -race -count=1)