Use XDG config directory as default config path

- Look for config at $XDG_CONFIG_HOME/converge/config.yaml by default
- --config still overrides to an explicit path
- Falls back to single-repo --repo mode if no config file found
- New defaultConfigPath and configFileExists helpers in Converge module

app/Main.hs

@@ -76,27 +76,40 @@ main :: IO ()
 main = do
     cli <- execParser parserInfo
     let mDebounceOverride = cliDebounce cli
-    options <- case cliConfig cli of
+    (options, configSource) <- case cliConfig cli of
         Just cfgPath -> do
             cfg <- loadConfig cfgPath
-            let opts = configToOptions cfg
-            pure $ case mDebounceOverride of
-                Just ms -> opts{optDebounceMs = ms}
-                Nothing -> opts
-        Nothing ->
-            pure $
-                defaultOptions
-                    { optRepos =
-                        [ defaultRepoConfig
-                            { rcPath = fromMaybe "." (cliRepo cli)
-                            , rcRemote = cliRemote cli
-                            , rcBranch = cliBranch cli
+            let opts = applyDebounce mDebounceOverride (configToOptions cfg)
+            pure (opts, Just cfgPath)
+        Nothing -> do
+            exists <- configFileExists
+            if exists
+                then do
+                    cfgPath <- defaultConfigPath
+                    cfg <- loadConfig cfgPath
+                    let opts = applyDebounce mDebounceOverride (configToOptions cfg)
+                    pure (opts, Just cfgPath)
+                else
+                    pure
+                        ( defaultOptions
+                            { optRepos =
+                                [ defaultRepoConfig
+                                    { rcPath = fromMaybe "." (cliRepo cli)
+                                    , rcRemote = cliRemote cli
+                                    , rcBranch = cliBranch cli
+                                    }
+                                ]
+                            , optDebounceMs = fromMaybe 5000 mDebounceOverride
                             }
-                        ]
-                    , optDebounceMs = fromMaybe 5000 mDebounceOverride
-                    }
+                        , Nothing
+                        )
+    case configSource of
+        Just path ->
+            hPutStrLn stderr ("Using config: " <> path)
+        Nothing ->
+            hPutStrLn stderr "No config file found, using single-repo mode"
     hPutStrLn stderr $
-        "Converge watching "
+        "Watching "
             <> show (length (optRepos options))
             <> " repo(s)"
             <> " (debounce: "
@@ -104,3 +117,7 @@ main = do
             <> "ms)"
     mapM_ (\r -> hPutStrLn stderr ("  - " <> rcPath r)) (optRepos options)
     runConverge options
+
+applyDebounce :: Maybe Int -> Options -> Options
+applyDebounce (Just ms) opts = opts{optDebounceMs = ms}
+applyDebounce Nothing opts = opts