feat: add Prometheus metrics for Store queries

Add InstrumentedStore decorator that wraps any Store and records
per-method query duration histograms and error counters. Wired into
main.go so all storage consumers get automatic observability.

Bump version to 0.18.0.

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

internal/metrics/metrics.go

@@ -25,6 +25,8 @@ type Metrics struct {
 	SessionDuration        prometheus.Histogram
 	ExecCommandsTotal      prometheus.Counter
 	BuildInfo              *prometheus.GaugeVec
+	StorageQueryDuration   *prometheus.HistogramVec
+	StorageQueryErrors     *prometheus.CounterVec
 }
 
 // New creates a new Metrics instance with all collectors registered.
@@ -79,6 +81,15 @@ func New(version string) *Metrics {
 			Name: "oubliette_build_info",
 			Help: "Build information. Always 1.",
 		}, []string{"version"}),
+		StorageQueryDuration: prometheus.NewHistogramVec(prometheus.HistogramOpts{
+			Name:    "oubliette_storage_query_duration_seconds",
+			Help:    "Duration of storage query calls in seconds.",
+			Buckets: []float64{0.001, 0.005, 0.01, 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5, 10},
+		}, []string{"method"}),
+		StorageQueryErrors: prometheus.NewCounterVec(prometheus.CounterOpts{
+			Name: "oubliette_storage_query_errors_total",
+			Help: "Total storage query errors.",
+		}, []string{"method"}),
 	}
 
 	reg.MustRegister(
@@ -95,6 +106,8 @@ func New(version string) *Metrics {
 		m.SessionDuration,
 		m.ExecCommandsTotal,
 		m.BuildInfo,
+		m.StorageQueryDuration,
+		m.StorageQueryErrors,
 	)
 
 	m.BuildInfo.WithLabelValues(version).Set(1)