fix: add required inputSchema to list_mailboxes tool

The MCP spec requires inputSchema on every tool. With the omitempty
JSON tag, the nil field was omitted entirely, causing clients to
reject the tools/list response and report 0 tools.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/mcpserver/server.go

@@ -150,6 +150,9 @@ func (s Server) Tools() []Tool {
 		{
 			Name:        "list_mailboxes",
 			Description: "List visible IMAP mailboxes for the configured account.",
+			InputSchema: map[string]any{
+				"type": "object",
+			},
 		},
 		{
 			Name:        "list_messages",