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/iostreams/iostreams.go

@@ -37,10 +37,10 @@ type IOStreams struct {
 }
 
 // New creates an IOStreams wired to the real os.Stdin, os.Stdout, and os.Stderr,
-// with TTY status auto-detected. Setting FGJ_FORCE_TTY=1 forces all streams to
-// be treated as TTYs.
+// with TTY status auto-detected. Setting FJ_FORCE_TTY=1 (or legacy FGJ_FORCE_TTY=1)
+// forces all streams to be treated as TTYs.
 func New() *IOStreams {
-	forceTTY := os.Getenv("FGJ_FORCE_TTY") != ""
+	forceTTY := os.Getenv("FJ_FORCE_TTY") != "" || os.Getenv("FGJ_FORCE_TTY") != ""
 
 	stdinTTY := forceTTY || (isTerminal(os.Stdin.Fd()))
 	stdoutTTY := forceTTY || (isTerminal(os.Stdout.Fd()))
@@ -118,14 +118,17 @@ func (s *IOStreams) ColorScheme() *ColorScheme {
 }
 
 // StartPager starts an external pager process and redirects Out to its stdin.
-// It checks FGJ_PAGER, then PAGER, then defaults to "less". If LESS is not
-// already set, it is set to "FRX" for a good default experience.
+// It checks FJ_PAGER (or legacy FGJ_PAGER), then PAGER, then defaults to "less".
+// If LESS is not already set, it is set to "FRX" for a good default experience.
 func (s *IOStreams) StartPager() error {
 	if !s.isStdoutTTY {
 		return nil
 	}
 
-	pagerCmd := os.Getenv("FGJ_PAGER")
+	pagerCmd := os.Getenv("FJ_PAGER")
+	if pagerCmd == "" {
+		pagerCmd = os.Getenv("FGJ_PAGER")
+	}
 	if pagerCmd == "" {
 		pagerCmd = os.Getenv("PAGER")
 	}