chore: remove the issue triaging agent

The agent ran on a schedule with a write-scoped GitHub token while feeding
attacker-controlled issue title and body straight into its prompt, so a
crafted issue could steer its labeling and owner-assignment actions.
Removing the agent and its workflow removes that exposure.

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 941289396

Author: GWeale