feat: centralize L402 config, automate Metal install, fix watchdog cleanup

- config.py: add L402_HMAC_SECRET, L402_MACAROON_SECRET, LIGHTNING_BACKEND
  to pydantic-settings with startup warnings for default secrets
- l402_proxy.py, mock_backend.py, factory.py: migrate from os.environ.get()
  to `from config import settings` per project convention
- Makefile: `make install-creative` now auto-installs PyTorch nightly with
  Metal (MPS) support on Apple Silicon instead of just printing a note
- activate_self_tdd.sh: add PID file (.watchdog.pid) and EXIT trap so
  Ctrl-C cleanly stops both the dashboard and the watchdog process
- .gitignore: add .watchdog.pid

https://claude.ai/code/session_01A81E5HMxZEPxzv2acNo35u

.gitignore

@@ -21,6 +21,9 @@ env/
 # SQLite memory — never commit agent memory
 *.db
 
+# Runtime PID files
+.watchdog.pid
+
 # Chat platform state files (contain bot tokens)
 telegram_state.json
 discord_state.json