feat: add log-level system to suppress noisy fallback debug output

Replace console.debug/log/warn calls in model router, retry, and daemon
startup with a structured logger that respects a configurable log_level.
Default level is 'info', suppressing verbose fallback debug messages in
the TUI while keeping them available via config when needed.

- Add src/logger.ts with debug/info/warn/error/silent levels
- Wire log_level into config schema (default: 'info')
- Initialize log level in both daemon and TUI startup paths
- Convert all console.debug in router.ts and retry.ts to logger.debug
- Convert console.log/warn in daemon/models.ts to logger.info/warn

config/default.yaml

@@ -1,6 +1,10 @@
 # Flynn Configuration
 # Copy to ~/.config/flynn/config.yaml and customize
 
+# Log verbosity: debug | info | warn | error | silent (default: info)
+# Set to 'debug' to see model fallback details.
+# log_level: info
+
 telegram:
   bot_token: ${FLYNN_TELEGRAM_TOKEN}
   allowed_chat_ids: []  # Add your Telegram chat ID