mavweb: add in-process auth gate for POST /tools

Add a local PasskeySession that handleTools checks before processing
any POST action (enable/disable). If the session hasn't been asserted
within the 5-minute TTL, return 403 Forbidden.

Changes:
- webauthn/session.go: add IsStepUp() convenience method (nil-safe)
- webauthn.go: PasskeyHandle holds a *PasskeySession; AssertFinish
  calls session.Assert() after IPC step-up
- main.go: create stepUpSession, pass to handleTools and
  newPasskeyHandle; handleTools returns 403 if !session.IsStepUp()
- handlers_test.go: update TestEnableTool_NoInProcessAuthGate to
  expect 403; add TestEnableTool_WithAuthGate_RequiresStepUp for
  the happy path with asserted session; update all 10 call sites

cmd/mavweb/main.go

@@ -142,12 +142,14 @@ func main() {
 	// Passkey registration + assertion are the step-up mechanism for
 	// AuthStepUp actions (tool enable). Without -webauthn-origin, these
 	// endpoints return 503 and step-up is unavailable (FloorSession).
+	stepUpSession := webauthn.NewPasskeySession(5 * time.Minute)
+
 	if *pkOrigin != "" && *pkRPID != "" && core != nil {
 		pk, err := newPasskeyHandle(webauthn.Config{
 			Origin: *pkOrigin,
 			RPID:   *pkRPID,
 			RPName: "maven",
-		}, core, *pkFile)
+		}, core, *pkFile, stepUpSession)
 		if err != nil {
 			log.Fatalf("passkey store: %v", err)
 		}
@@ -163,7 +165,7 @@ func main() {
 	// Enabling is the boundary-moving act (maven.md), so it lives ONLY here,
 	// behind wg+nginx+auth — never the voice/chat path.
 	mux.HandleFunc("/tools", func(w http.ResponseWriter, r *http.Request) {
-		handleTools(w, r, core)
+		handleTools(w, r, core, stepUpSession)
 	})
 
 	srv := &http.Server{Addr: *addr, Handler: mux}
@@ -425,7 +427,7 @@ func handleRevert(w http.ResponseWriter, r *http.Request, core ipc.CoreAPI) {
 // whitespace-split — argv with embedded spaces isn't supported (ponytail: no
 // shell-word parsing; the box owner controls this input, quote a wrapper script
 // if an arg needs spaces).
-func handleTools(w http.ResponseWriter, r *http.Request, core ipc.CoreAPI) {
+func handleTools(w http.ResponseWriter, r *http.Request, core ipc.CoreAPI, session *webauthn.PasskeySession) {
 	if core == nil {
 		http.Error(w, "tools disabled (no -core)", http.StatusServiceUnavailable)
 		return
@@ -433,6 +435,10 @@ func handleTools(w http.ResponseWriter, r *http.Request, core ipc.CoreAPI) {
 	ctx := r.Context()
 	var msg string
 	if r.Method == http.MethodPost {
+		if !session.IsStepUp() {
+			http.Error(w, "step-up required: assert a passkey first", http.StatusForbidden)
+			return
+		}
 		action := r.FormValue("action")
 		name := strings.TrimSpace(r.FormValue("name"))
 		switch action {