fix: use exec.LookPath instead of 'command -v' for binary detection

'command' is a shell builtin, not a real binary, so exec.Command
always failed with exit 127. Replaced with os/exec.LookPath which
correctly resolves binaries in PATH.

Also adds capabilities_test.go to verify Probe() matches
exec.LookPath results.

Version: 0.7.2

internal/storage/capabilities_test.go

@@ -0,0 +1,49 @@
+package storage
+
+import (
+	"context"
+	"os/exec"
+	"testing"
+)
+
+func TestProbeDetectsRsync(t *testing.T) {
+	caps, err := Probe(context.Background())
+	if err != nil {
+		t.Fatalf("Probe: %v", err)
+	}
+	_, rsyncErr := exec.LookPath("rsync")
+	if rsyncErr != nil && caps.Rsync {
+		t.Error("Probe claims rsync is available but exec.LookPath says it is not")
+	}
+	if rsyncErr == nil && !caps.Rsync {
+		t.Error("exec.LookPath found rsync but Probe did not detect it")
+	}
+}
+
+func TestProbeDetectsSnapraid(t *testing.T) {
+	caps, err := Probe(context.Background())
+	if err != nil {
+		t.Fatalf("Probe: %v", err)
+	}
+	_, snapraidErr := exec.LookPath("snapraid")
+	if snapraidErr != nil && caps.SnapraidBin {
+		t.Error("Probe claims snapraid is available but exec.LookPath says it is not")
+	}
+	if snapraidErr == nil && !caps.SnapraidBin {
+		t.Error("exec.LookPath found snapraid but Probe did not detect it")
+	}
+}
+
+func TestProbeDetectsMergerfsBin(t *testing.T) {
+	caps, err := Probe(context.Background())
+	if err != nil {
+		t.Fatalf("Probe: %v", err)
+	}
+	_, mergerfsErr := exec.LookPath("mergerfs")
+	if mergerfsErr != nil && caps.MergerfsBin {
+		t.Error("Probe claims mergerfs is available but exec.LookPath says it is not")
+	}
+	if mergerfsErr == nil && !caps.MergerfsBin {
+		t.Error("exec.LookPath found mergerfs but Probe did not detect it")
+	}
+}