fix: suppress all compiler warnings

- mcp_server.rs: #[allow(dead_code)] on NotificationCallback type alias and get_notification_sender method
- handlers/mod.rs: #[allow(dead_code)] on past_complete_month helper
- Cargo.toml: move maintenance key into [package.metadata] (was invalid at package level)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/tools/handlers/mod.rs

@@ -179,6 +179,7 @@ pub(crate) fn dir_size(path: &Path) -> u64 {
         .sum()
 }
 
+#[allow(dead_code)]
 pub(crate) fn past_complete_month() -> (String, String) {
     let now = chrono::Utc::now();
     let today = chrono::NaiveDate::from_ymd_opt(now.year(), now.month(), 1)