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/manpages.go

@@ -29,7 +29,7 @@ var manpagesCmd = &cobra.Command{
 		}
 
 		header := &doc.GenManHeader{
-			Title:   "FGJ",
+			Title:   "FJ",
 			Section: "1",
 		}