controller v0.67.1: gate only acts on external drives under /mnt/felhom-drives/

Fix caught live: planDriveGates falsely marked the internal SSD path
/mnt/sys_drive/felhom-data disconnected (agent never reports it), which would
block starting SSD-resident apps. Gate now skips non-/mnt/felhom-drives/ paths.
Regression case added. No apps were stopped (none depended on the SSD path).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

controller/internal/web/intermediary_test.go

@@ -38,6 +38,7 @@ func TestPlanDriveGates(t *testing.T) {
 		{Path: "/mnt/felhom-drives/back", Disconnected: true},     // present + disconnected → RETURN
 		{Path: "/mnt/felhom-drives/gone", Disconnected: true},     // ABSENT + disconnected → no action (steady)
 		{Path: "/mnt/felhom-drives/dead", Decommissioned: true},   // decommissioned → never touched
+		{Path: "/mnt/sys_drive/felhom-data"},                      // INTERNAL SSD (absent from agent) → never gated
 	}
 	disks := []agentapi.DiskInfo{
 		{MountPath: "/mnt/usb", GuestPath: "/mnt/felhom-drives/usb", State: "attached"},
@@ -66,4 +67,7 @@ func TestPlanDriveGates(t *testing.T) {
 	if _, acted := actions["/mnt/felhom-drives/dead"]; acted {
 		t.Errorf("decommissioned drive must never be gated")
 	}
+	if _, acted := actions["/mnt/sys_drive/felhom-data"]; acted {
+		t.Errorf("internal SSD/system path must NEVER be gated (only /mnt/felhom-drives/ externals)")
+	}
 }