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

test/TestHelper.hs

@@ -16,12 +16,13 @@ module TestHelper (
 ) where
 
 import Control.Concurrent.MVar (newMVar)
-import Converge (Logger (..))
+import Converge (Logger (..), LogLevel (..))
 import Data.Text (Text)
 import qualified Data.Text as T
 import System.Directory (createDirectory)
 import System.Exit (ExitCode)
 import System.FilePath ((</>))
+import System.IO (stderr)
 import System.IO.Temp (withSystemTempDirectory)
 import System.Process (readProcessWithExitCode)
 
@@ -37,9 +38,9 @@ rawGit args = do
     (code, out, err) <- readProcessWithExitCode "git" args ""
     pure (code, T.pack out, T.pack err)
 
--- | Create a test logger (serialized writes to stderr).
+-- | Create a test logger (serialized writes to stderr, Debug level).
 testLogger :: IO Logger
-testLogger = Logger <$> newMVar ()
+testLogger = Logger <$> newMVar () <*> pure Debug <*> pure stderr
 
 {- | Create a temporary git repository, configure a user, make an initial
 commit on branch "main", run the action with the repo path, then clean up.