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]

internal/config/config.go

@@ -131,7 +131,7 @@ func (c *Config) SaveToPath(path string) error {
 // Priority order:
 // 1. Explicitly provided hostname parameter
 // 2. CLI flag (--hostname)
-// 3. Environment variable (FGJ_HOST)
+// 3. Environment variable (FJ_HOST, with FGJ_HOST fallback)
 // 4. Auto-detected hostname from git remote
 // 5. match_dirs lookup (longest prefix match)
 // 6. Default to codeberg.org
@@ -141,7 +141,7 @@ func (c *Config) GetHost(hostname string, detectedHost string, cwd string) (Host
 	}
 
 	if hostname == "" {
-		hostname = os.Getenv("FGJ_HOST")
+		hostname = EnvWithFallback("FJ_HOST", "FGJ_HOST")
 	}
 
 	if hostname == "" {
@@ -228,6 +228,15 @@ func (c *Config) ResolveHostByPath(cwd string) string {
 }
 
 // expandHome replaces a leading ~ with the user's home directory.
+// EnvWithFallback returns the value of the primary env var, falling back to
+// the legacy name if the primary is unset. This eases the FGJ_ → FJ_ rename.
+func EnvWithFallback(primary, legacy string) string {
+	if v := os.Getenv(primary); v != "" {
+		return v
+	}
+	return os.Getenv(legacy)
+}
+
 func expandHome(path string) string {
 	if path == "~" || strings.HasPrefix(path, "~/") {
 		home, err := os.UserHomeDir()