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>

tests/test_assistant.py

@@ -411,3 +411,55 @@ def test_transcription_falls_back_to_one_upload():
     t.add([frame(LOUD)])  # nowhere to go: the live socket never opened
     assert t.finish(frame(LOUD) * 2) == "hey bolt"
     assert b"".join(late.audio) == frame(LOUD) * 2
+
+
+# ── heartbeat announcements ──────────────────────────────────────────────────
+
+def test_announcements_are_spoken_when_idle():
+    a, asked, played, logs, _ = make([])
+    a.announce("You have a reminder.")
+    a._drain_announcements()
+    assert "You have a reminder." in spoken(played)
+    assert a.state == "idle"
+
+
+def test_announcements_wait_for_idle():
+    a, asked, played, logs, _ = make(["It's three o'clock."])
+    a.state = "busy"
+    a.announce("You have a reminder.")
+    a._drain_announcements()
+    assert spoken(played) == []  # held back while Bolt is talking
+    a.state = "idle"
+    a._drain_announcements()
+    assert "You have a reminder." in spoken(played)
+
+
+def test_announcements_hold_while_a_transcript_is_in_flight():
+    a, asked, played, logs, _ = make([])
+    a._pending.append(object())  # an utterance is still at the server
+    a.announce("You have a reminder.")
+    a._drain_announcements()
+    assert spoken(played) == []
+    a._pending.clear()
+    a._drain_announcements()
+    assert "You have a reminder." in spoken(played)
+
+
+def test_announcements_hold_while_someone_is_speaking():
+    a, asked, played, logs, _ = make([])
+    FakeTranscription.script.append("")  # the start event still mints a transcriber
+    for f in [frame(LOUD)] * 3:
+        a.process(f)  # the segmenter is mid-utterance
+    a.announce("You have a reminder.")
+    a._drain_announcements()
+    assert spoken(played) == []
+    a.segmenter.reset()
+    a._drain_announcements()
+    assert "You have a reminder." in spoken(played)
+
+
+def test_blank_announcements_are_dropped():
+    a, asked, played, logs, _ = make([])
+    a.announce("   ")
+    a._drain_announcements()
+    assert spoken(played) == []