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>

README.md

@@ -45,6 +45,11 @@ other than the system default. `run.bat --type` lets you type instead of speak,
    without needing the wake phrase again.
 4. **While Bolt speaks, the mic is ignored**, so his own voice through the speakers (which often
    says "Bolt") can't wake him.
+5. **Reminders and delivered files ride a heartbeat.** Every `HEARTBEAT_SECONDS` (default 60) the
+   assistant reports in to `/desk/report_status`, which is how Bolt's reminders and any files he
+   sends to this client arrive — the same channel Bolt-Pet uses. Announcements are spoken when the
+   mic is idle, and files download to `BOLT_DOWNLOAD_DIR` (default `~/Downloads/Bolt`), never
+   overwriting what's already there. Set `HEARTBEAT_SECONDS=0` to turn it off.
 
 ## Cost and privacy
 
@@ -85,6 +90,9 @@ See `.env.example`. The ones you're most likely to change:
   (default `assistant`: "what's the weather, assistant?"). Blank to turn off.
 - `COMMAND_WAIT_SEC`, `FOLLOW_UP_SEC`: how long it waits after "Hey Bolt", or after asking you
   something.
+- `HEARTBEAT_SECONDS`: how often it checks in for reminders and delivered files (default 60;
+  `0` turns the heartbeat off).
+- `BOLT_DOWNLOAD_DIR`: where delivered files land (default `~/Downloads/Bolt`).
 
 It has its own conversation on the server (`pet-assistant-<computer>`), which shows up in Bolt
 Console's Chats list.