Add configurable log levels and log file support

- New LogLevel type: Debug, Info, Warn, Error with Ord instance
- Logger holds configured level and output handle (stderr or file)
- --log-level and --log-file CLI flags
- log_level and log_file YAML config fields
- Log messages assigned appropriate severity levels
- Test logger updated to new Logger structure
- All 17 tests passing

converge.example.yaml

@@ -11,3 +11,6 @@ repos:
     # branch defaults to "main"
 
 # debounce_ms: 5000   # optional, defaults to 5000
+
+# log_level: info      # optional, one of: debug, info, warn, error (default: info)
+# log_file: /var/log/converge.log   # optional, write logs to a file instead of stderr