complete fgj → fj rename: env vars, config migration, docs

- Rename env vars: FGJ_HOST → FJ_HOST, FGJ_TOKEN → FJ_TOKEN,
  FGJ_FORCE_TTY → FJ_FORCE_TTY, FGJ_PAGER → FJ_PAGER,
  FGJ_BINARY_PATH → FJ_BINARY_PATH (all with legacy fallback)
- Auto-migrate ~/.config/fgj/ → ~/.config/fj/ on first run
- Update man page title, README, CHANGELOG
- Update test fixture labels from [FGJ E2E Test] to [FJ E2E Test]

cmd/root.go

@@ -2,7 +2,9 @@ package cmd
 
 import (
 	"fmt"
+	"io"
 	"os"
+	"path/filepath"
 	"strconv"
 	"strings"
 
@@ -52,6 +54,18 @@ func initConfig() {
 		}
 
 		configDir := home + "/.config/fj"
+		legacyDir := home + "/.config/fgj"
+
+		// Migrate from ~/.config/fgj/ if the new dir doesn't exist yet.
+		if _, err := os.Stat(configDir); os.IsNotExist(err) {
+			if info, err := os.Stat(legacyDir); err == nil && info.IsDir() {
+				if copyErr := migrateConfigDir(legacyDir, configDir); copyErr == nil {
+					fmt.Fprintln(ios.ErrOut, "notice: migrated config from ~/.config/fgj/ to ~/.config/fj/")
+					fmt.Fprintln(ios.ErrOut, "        you can remove ~/.config/fgj/ when ready")
+				}
+			}
+		}
+
 		_ = os.MkdirAll(configDir, 0755)
 
 		viper.AddConfigPath(configDir)
@@ -60,7 +74,7 @@ func initConfig() {
 	}
 
 	viper.AutomaticEnv()
-	viper.SetEnvPrefix("FGJ")
+	viper.SetEnvPrefix("FJ")
 
 	_ = viper.ReadInConfig()
 }
@@ -127,3 +141,35 @@ func parseIssueArg(arg string) (int64, error) {
 	}
 	return strconv.ParseInt(arg, 10, 64)
 }
+
+// migrateConfigDir copies all files from src to dst (one level, no subdirs).
+func migrateConfigDir(src, dst string) error {
+	if err := os.MkdirAll(dst, 0755); err != nil {
+		return err
+	}
+	entries, err := os.ReadDir(src)
+	if err != nil {
+		return err
+	}
+	for _, e := range entries {
+		if e.IsDir() {
+			continue
+		}
+		in, err := os.Open(filepath.Join(src, e.Name()))
+		if err != nil {
+			return err
+		}
+		out, err := os.OpenFile(filepath.Join(dst, e.Name()), os.O_CREATE|os.O_WRONLY, 0600)
+		if err != nil {
+			in.Close()
+			return err
+		}
+		_, err = io.Copy(out, in)
+		in.Close()
+		out.Close()
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}