feat(routing): decision policy

Pure-function Policy{Floor,Ceil} with Decide(*float64, uint64) Decision.
Rules in priority order: nil → local; ≥floor → local; <ceil → claude;
sample band → low bit of requestHash.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/routing/policy.go

@@ -0,0 +1,47 @@
+package routing
+
+// Decision is the route picked for a single skill call.
+type Decision int
+
+const (
+	DecideLocal Decision = iota
+	DecideClaude
+)
+
+func (d Decision) String() string {
+	if d == DecideLocal {
+		return "local"
+	}
+	return "claude"
+}
+
+// Policy holds the floor/ceil thresholds for routing decisions.
+//
+// Rules (in order):
+//
+//  1. passRate == nil           → DecideLocal (default-to-local for cost-routable skills)
+//  2. *passRate >= Floor        → DecideLocal (trust local)
+//  3. *passRate <  Ceil         → DecideClaude (don't trust local)
+//  4. otherwise (sample band)   → requestHash low bit picks: 0=local, 1=claude
+type Policy struct {
+	Floor float64
+	Ceil  float64
+}
+
+// Decide returns the routing decision for a single call.
+// requestHash is consulted only when passRate is in the sample band [Ceil, Floor).
+func (p Policy) Decide(passRate *float64, requestHash uint64) Decision {
+	if passRate == nil {
+		return DecideLocal
+	}
+	if *passRate >= p.Floor {
+		return DecideLocal
+	}
+	if *passRate < p.Ceil {
+		return DecideClaude
+	}
+	if requestHash&1 == 0 {
+		return DecideLocal
+	}
+	return DecideClaude
+}