feat(tools): refactor pushover into conditional notify tool with Telegram fallback

Replace the always-registered PushoverTool with a NotifyTool that
auto-selects its backend at startup: Pushover if .env credentials exist,
otherwise Telegram (using bot_token + first allowed_users entry as
chat_id). If neither backend is available, the tool is not registered,
saving a tool slot and avoiding agent confusion.

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

src/config/schema.rs

@@ -31,7 +31,7 @@ const SUPPORTED_PROXY_SERVICE_KEYS: &[&str] = &[
     "tool.browser",
     "tool.composio",
     "tool.http_request",
-    "tool.pushover",
+    "tool.notify",
     "memory.embeddings",
     "tunnel.custom",
 ];