DET-004: Refactor more Policy Gates for determinism

- BudgetConstraintEnforcer: inject TimeProvider
- EvidenceFreshnessGate: inject TimeProvider

Replace DateTimeOffset.UtcNow with _timeProvider.GetUtcNow()

Sprint: SPRINT_20260104_001_BE_determinism_timeprovider_injection

src/Policy/__Libraries/StellaOps.Policy/Gates/BudgetConstraintEnforcer.cs

@@ -11,14 +11,17 @@ public sealed class BudgetConstraintEnforcer : IBudgetConstraintEnforcer
     private readonly IBudgetLedger _ledger;
     private readonly IGateSelector _gateSelector;
     private readonly ILogger<BudgetConstraintEnforcer> _logger;
+    private readonly TimeProvider _timeProvider;
 
     public BudgetConstraintEnforcer(
         IBudgetLedger ledger,
         IGateSelector gateSelector,
+        TimeProvider? timeProvider = null,
         ILogger<BudgetConstraintEnforcer>? logger = null)
     {
         _ledger = ledger ?? throw new ArgumentNullException(nameof(ledger));
         _gateSelector = gateSelector ?? throw new ArgumentNullException(nameof(gateSelector));
+        _timeProvider = timeProvider ?? TimeProvider.System;
         _logger = logger ?? NullLogger<BudgetConstraintEnforcer>.Instance;
     }
 
@@ -134,7 +137,7 @@ public sealed class BudgetConstraintEnforcer : IBudgetConstraintEnforcer
             TotalRiskPoints = penaltyRiskPoints,
             Budget = consumeResult.Budget,
             FollowUpRequired = true,
-            FollowUpDeadline = DateTimeOffset.UtcNow.AddDays(5)
+            FollowUpDeadline = _timeProvider.GetUtcNow().AddDays(5)
         };
     }
 

src/Policy/__Libraries/StellaOps.Policy/Gates/EvidenceFreshnessGate.cs

@@ -13,13 +13,16 @@ public sealed class EvidenceFreshnessGate : IPolicyGate
 {
     private readonly IEvidenceTtlEnforcer _ttlEnforcer;
     private readonly ILogger<EvidenceFreshnessGate> _logger;
+    private readonly TimeProvider _timeProvider;
 
     public EvidenceFreshnessGate(
         IEvidenceTtlEnforcer ttlEnforcer,
-        ILogger<EvidenceFreshnessGate> logger)
+        ILogger<EvidenceFreshnessGate> logger,
+        TimeProvider? timeProvider = null)
     {
         _ttlEnforcer = ttlEnforcer;
         _logger = logger;
+        _timeProvider = timeProvider ?? TimeProvider.System;
     }
 
     public Task<GateResult> EvaluateAsync(
@@ -30,7 +33,7 @@ public sealed class EvidenceFreshnessGate : IPolicyGate
         // Build evidence bundle from context
         var evidenceBundle = BuildEvidenceBundleFromContext(context);
 
-        var freshnessResult = _ttlEnforcer.CheckFreshness(evidenceBundle, DateTimeOffset.UtcNow);
+        var freshnessResult = _ttlEnforcer.CheckFreshness(evidenceBundle, _timeProvider.GetUtcNow());
 
         var details = ImmutableDictionary.CreateBuilder<string, object>();
         details.Add("overall_status", freshnessResult.OverallStatus.ToString());