cancel_thread: new mcp tool — unify reminder + question cancel on both surfaces

hive-c0re/src/broker.rs

@@ -405,6 +405,40 @@ impl Broker {
         Ok(n)
     }
 
+    /// Cancel a pending reminder on behalf of `canceller`. Returns
+    /// the owner agent name on success (handy for logging). Auth
+    /// rules mirror `OperatorQuestions::cancel`: owner, operator, or
+    /// manager.
+    pub fn cancel_reminder_as(&self, id: i64, canceller: &str) -> Result<String> {
+        let conn = self.conn.lock().unwrap();
+        let owner: Option<String> = conn
+            .query_row(
+                "SELECT agent FROM reminders WHERE id = ?1 AND sent_at IS NULL",
+                params![id],
+                |row| row.get(0),
+            )
+            .optional()?;
+        let Some(owner) = owner else {
+            anyhow::bail!("reminder {id} not pending (already delivered or unknown)");
+        };
+        let authorised = canceller == owner
+            || canceller == hive_sh4re::OPERATOR_RECIPIENT
+            || canceller == hive_sh4re::MANAGER_AGENT;
+        if !authorised {
+            anyhow::bail!(
+                "reminder {id}: '{canceller}' not allowed to cancel (owner = '{owner}')"
+            );
+        }
+        let n = conn.execute(
+            "DELETE FROM reminders WHERE id = ?1 AND sent_at IS NULL",
+            params![id],
+        )?;
+        if n == 0 {
+            anyhow::bail!("reminder {id} vanished between auth check and delete");
+        }
+        Ok(owner)
+    }
+
     /// Get up to `limit` due reminders across all agents in a single query.
     /// Returns `(agent, id, message, file_path)` tuples. Pass a small limit
     /// (e.g. 100) so a burst of overdue reminders doesn't flood the broker