feat(TASK-049): harden hook dispatch against NUL env values and nested-supervisor latch misses

- strip interior NUL bytes from hook env values in fire_resolved so
  agent-authored text (e.g. escalation questions) cannot suppress a hook
  via a failed spawn; raise the spawn-failure log to warn!
- make Supervisor::has_active_tasks recurse into child supervisors,
  mirroring cancel_recursive, so a prompt ctrl-c that cancels only a
  grandchild still latches as an interruption
- document the signal-first vs error-first classification asymmetry at
  top_level_agent_finished / SpawnResultHooks::fire and the deliberate
  REPL teardown cancel; pin ctrlc-with-no-error => agent.interrupted
- probe Scenario J: pty-driven ctrl-c during a live headless --agent run
  fires exactly one agent.interrupted with no COYOTE_AGENT_ERROR

src/repl/mod.rs

@@ -527,6 +527,9 @@ Type ".help" for additional help.
         }
 
         if let Some(supervisor) = self.ctx.read().supervisor.clone() {
+            // Leaving the loop is a deliberate exit, never an interruption:
+            // cancelling straggler children here does not latch the abort
+            // signal, so teardown still classifies as agent.completed.
             supervisor.read().cancel_recursive();
         }