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/assistant.py

@@ -20,6 +20,10 @@ background thread, so the mic keeps being segmented meanwhile and whatever
 is said next becomes its own utterance and is sent off too. Finished
 transcripts are delivered oldest-first between mic frames.
 
+Announcements (Bolt's reminders, delivered-file notices) ride the heartbeat
+thread and wait in a queue; the run loop speaks them one at a time whenever
+it is idle, then flushes the echo of Bolt's own voice like after a reply.
+
 Everything that touches hardware or the network is injected, so the flow
 is tested with fakes; __main__ wires in the real mic, server and speaker.
 """
@@ -115,6 +119,7 @@ class Assistant:
         self._mute_until = 0.0
         self._voice = ""  # a voice the server picked (speak_as) sticks for later replies
         self._tts_failed = False
+        self._announcements: "queue.Queue[str]" = queue.Queue()  # heartbeat notices, spoken when idle
 
     # ── audio in ─────────────────────────────────────────────────────────
 
@@ -135,6 +140,7 @@ class Assistant:
             else:
                 self.process(frame)
             self.handle_transcripts()
+            self._drain_announcements()
 
     def listening_banner(self) -> str:
         if not self.wake_required:
@@ -208,6 +214,29 @@ class Assistant:
                 self.log("  (stopped waiting)")
             self.state = "idle"
 
+    # ── heartbeat announcements ──────────────────────────────────────────
+
+    def announce(self, text: str) -> None:
+        """Queue something to say once the room is free — a reminder Bolt
+        fired, a delivered-file notice. Never interrupts: it waits for idle."""
+        text = (text or "").strip()
+        if text:
+            self._announcements.put(text)
+
+    def _drain_announcements(self) -> None:
+        """Say what the heartbeat brought, one at a time, while idle — never
+        mid-sentence, while a transcript is still in flight, or during the
+        echo guard after Bolt's own voice."""
+        if (self.state != "idle" or self._pending or self.segmenter.in_speech
+                or self.clock() < self._mute_until):
+            return
+        try:
+            text = self._announcements.get_nowait()
+        except queue.Empty:
+            return
+        self.speak(text)
+        self._back_to_listening("idle")  # flush the mic's echo of Bolt's own voice
+
     # ── what was said ────────────────────────────────────────────────────
 
     def heard(self, text: str) -> None: