Make delivery and integration failures explicit

Persist reminder presentations and retry state, atomically complete collapsed deliveries, fall back across away reaches, and block permanent failures visibly (V-715, V-678). Fail closed when enabled integrations lack credentials and keep remote arms explicitly dark (V-691). Give mavweb one sanitized, request-correlated error contract (V-689). Owner explicitly requested direct commits to master.

cmd/mavweb/shell.go

@@ -159,9 +159,10 @@ func renderPage(w http.ResponseWriter, t *template.Template, data any) {
 // requireCore answers whether the surface has a core to read. mavweb runs
 // without -core (voice-only), and every page that needs mavend says so with a
 // 503 naming itself rather than a blank error.
-func requireCore(w http.ResponseWriter, core ipc.CoreAPI, surface string) bool {
+func requireCore(w http.ResponseWriter, r *http.Request, core ipc.CoreAPI, surface string) bool {
 	if core == nil {
-		http.Error(w, surface+" disabled (no -core)", http.StatusServiceUnavailable)
+		writeProblem(w, r, http.StatusServiceUnavailable, problemCoreUnavailable,
+			surface+" disabled (no -core)", nil)
 		return false
 	}
 	return true
@@ -169,11 +170,12 @@ func requireCore(w http.ResponseWriter, core ipc.CoreAPI, surface string) bool {
 
 // stepUpGate reports whether the caller may proceed through the AuthStepUp
 // gate, writing the 403 itself when it may not. See stepUpOK for the policy.
-func stepUpGate(w http.ResponseWriter, session *webauthn.PasskeySession, requireStepUp bool) bool {
+func stepUpGate(w http.ResponseWriter, r *http.Request, session *webauthn.PasskeySession, requireStepUp bool) bool {
 	if stepUpOK(session, requireStepUp) {
 		return true
 	}
-	http.Error(w, "step-up required: assert a passkey first", http.StatusForbidden)
+	writeProblem(w, r, http.StatusForbidden, problemStepUpRequired,
+		"step-up required: assert a passkey first", nil)
 	return false
 }