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>

.gitignore

@@ -3,3 +3,4 @@ __pycache__/
 *.pyc
 .env
 .pytest_cache/
+.claude/