Let an own secret's manifest declare a desk prompt (issue 789)

A family like mesh-mail's mail-credential-* names only the module and the secret's key at the desk, so the operator cannot tell which account is asked. An own secret (or a family) may now declare a one-line prompt in its manifest, with {member} filled by a family member's suffix; the module check refuses one that is not one line, over 120 characters, or holds another placeholder. desk_secret.go reads it from the controller's own catalogue and sends it to the desk as what, never from the call or the module's runtime; omitted when none is declared, so an old desk or module behaves as today.

internal/inventory/secrets.go

@@ -595,6 +595,26 @@ func (i *Inventory) DeclaresOwnSecret(ctx context.Context, module, name string)
 	return GivableAtDesk(m, name)
 }
 
+// OwnSecretPrompt is the one line the desk asks for this secret as — the module's declared `prompt`, read from
+// the controller's own catalogue, never from the call or the module's runtime (novox/hq issue 789). For a family
+// member it is the family's prompt with `{member}` replaced by the member's suffix. Empty when the module or the
+// secret is unknown, or declares no prompt — the desk then asks by name alone, as it does today.
+func (i *Inventory) OwnSecretPrompt(ctx context.Context, module, name string) (string, error) {
+	m, err := i.declared(ctx, module)
+	if err != nil {
+		return "", err
+	}
+	own, family, ok := m.OwnSecrets.Lookup(name)
+	if !ok || own.Prompt == "" {
+		return "", nil
+	}
+	if family == "" {
+		return own.Prompt, nil
+	}
+	member := strings.TrimPrefix(name, catalogue.FamilyPrefix(family))
+	return strings.ReplaceAll(own.Prompt, "{member}", member), nil
+}
+
 // RunsAsItsOwnAccount says a module runs as an account of its own (novox/hq ADR 0259 §8): a trusted party — the
 // router, a channel that proves its sender or shows a link's code — whose own secret is what the operator's
 // answers are believed by. Its value is given at the controller's terminal alone.