Add heartbeat: reminders and delivered files reach Bolt Assistant

The client never called /desk/report_status, so Bolt's reminders and any
files he delivered to this session sat in the server's outbox forever -
the same dead-air failure the pet client showed when its heartbeat stopped.
Now a heartbeat thread (HEARTBEAT_SECONDS, default 60; 0 disables) reports
in, speaks announcements through the assistant's queue when the mic is
idle, and downloads the outbox to BOLT_DOWNLOAD_DIR (default
~/Downloads/Bolt) via .part-then-rename, never overwriting. A startup
health probe warns loudly when the server refuses the DESK_API_KEY.

Also: add the .env.example the README's setup step references (it was
missing, so `cp .env.example .env` failed for everyone), and gitignore
.claude/ so Claude Code's worktree directory stays out of the repo.

Tests: tests/test_heartbeat.py (12) plus announcement tests in
tests/test_assistant.py (6); full suite 135 passed.

Co-Authored-By: Claude Code <noreply@anthropic.com>

bolt_assistant/__main__.py

@@ -99,6 +99,25 @@ def main(argv=None) -> int:
             return 0
 
     stop = threading.Event()
+    from . import heartbeat
+
+    heartbeat.fetch_server_health()
+
+    def _on_heartbeat(result: dict) -> None:
+        # What the server hands out is popped the moment it is handed over, so
+        # everything goes into the spoken queue — announced when the mic is idle.
+        if result.get("announcement"):
+            assistant.announce(result["announcement"])
+        saved = result.get("saved") or []
+        if saved:
+            names = ", ".join(path.name for path in saved)
+            folder = str(saved[0].parent)
+            assistant.announce(
+                f"Saved {names} to {folder}."
+                if len(saved) <= 3 else f"Saved {len(saved)} files to {folder}.")
+
+    threading.Thread(target=heartbeat.heartbeat_loop, args=(stop, _on_heartbeat),
+                     name="bolt-heartbeat", daemon=True).start()
     try:
         mic = audio.open_mic(assistant.on_mic_frame)
     except Exception as exc: