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]

tests/functional/fixtures.go

@@ -230,7 +230,10 @@ func (env *TestEnv) CleanupRepo(owner, repoName string) {
 
 // GetBinaryPath returns the path to the built fj binary
 func (env *TestEnv) GetBinaryPath() string {
-	binaryPath := os.Getenv("FGJ_BINARY_PATH")
+	binaryPath := os.Getenv("FJ_BINARY_PATH")
+	if binaryPath == "" {
+		binaryPath = os.Getenv("FGJ_BINARY_PATH")
+	}
 	if binaryPath == "" {
 		// Look for the binary in common locations
 		candidates := []string{