feat(tools): add native Pushover tool with priority and sound support

- Implements Pushover API as native tool (reqwest-based)
- Supports message, title, priority (-2 to 2), sound parameters
- Reads credentials from .env file in workspace
- 11 comprehensive tests covering schema, credentials, edge cases
- Follows CONTRIBUTING.md tool implementation patterns

src/channels/mod.rs

@@ -852,6 +852,10 @@ pub async fn start_channels(config: Config) -> Result<()> {
         "schedule",
         "Manage scheduled tasks (create/list/get/cancel/pause/resume). Supports recurring cron and one-shot delays.",
     ));
+    tool_descs.push((
+        "pushover",
+        "Send a Pushover notification to your device. Requires PUSHOVER_TOKEN and PUSHOVER_USER_KEY in .env file.",
+    ));
     if !config.agents.is_empty() {
         tool_descs.push((
             "delegate",