refactor(heartbeat): remove Completed section and tighten section gating

- Remove ## Completed section from HEARTBEAT.md template; completed
  tasks should be deleted, not accumulated
- Change in_active_section from tri-state (None/True/False) to bool
  (True/False) so stray text before any ## heading no longer triggers
  heartbeat
- Add test cases for stray pre-heading text and ## Notes section
- Update docs/chat-commands.md to reference ## Active Tasks

nanobot/cli/commands.py

@@ -107,7 +107,7 @@ _HEARTBEAT_PREAMBLE = (
 def _heartbeat_has_active_tasks(content: str) -> bool:
     """True if HEARTBEAT.md has task lines, ignoring headers, blanks and comments."""
     in_comment = False
-    in_active_section: bool | None = None
+    in_active_section: bool = False
     for line in content.splitlines():
         stripped = line.strip()
         if in_comment: