fix: start iris in multiplexer panes instead of inheriting a dead one (#120)

closes #62
when you open a new Zellij pane, the shell inherits Iris’s environment
variables, so it thinks Iris is already running. But the pane doesn't
inherit the underlying connection (FD 13), so suggestions silently fail
into a dead pipe

the old code only checked for `tmux` by process name, ignoring Zellij
completely

- Fixed:

instead of checking for specific app names, Iris now tracks the active
terminal device (`IRIS_TTY`)

if a new shell detects that it's on a different `TTY` than Iris, it
clears the stale environment variables so Iris can start fresh. The
`IRIS_PID != PPID` check prevents Iris from trying to re-exec itself
when spawning sub-shells.

```sh
if [ -n "$IRIS_PID" ] && [ "$IRIS_PID" != "$PPID" ] && [ "${TTY:-$(tty 2>/dev/null)}" != "$IRIS_TTY" ]; then
    unset IRIS_PID IRIS_IS_CHILD IRIS_FD IRIS_TTY
fi

```

root/wrapper.go

@@ -153,15 +153,27 @@ func runWrapper() {
 	c.ExtraFiles = make([]*os.File, 11)
 	// pass write end of pipe to shell as fd 13 (since index 10 maps to 13)
 	c.ExtraFiles[10] = w
-	c.Env = adapter.GetEnv(13, os.Getpid())
 
-	ptmx, err := pty.Start(c)
+	ptmx, tts, err := pty.Open()
 	if err != nil {
-		_, _ = fmt.Fprintf(os.Stderr, "[IRIS] failed to start PTY: %v\n", err)
+		_, _ = fmt.Fprintf(os.Stderr, "[IRIS] failed to open PTY: %v\n", err)
 		return
 	}
 	defer func() { _ = ptmx.Close() }()
 
+	c.Stdin, c.Stdout, c.Stderr = tts, tts, tts
+	c.SysProcAttr = &syscall.SysProcAttr{Setsid: true, Setctty: true}
+	// the shell compares its own tty against this to tell whether the inherited
+	// IRIS_* vars belong to it or leaked in from an outer terminal
+	c.Env = append(adapter.GetEnv(13, os.Getpid()), "IRIS_TTY="+tts.Name())
+
+	if err = c.Start(); err != nil {
+		_ = tts.Close()
+		_, _ = fmt.Fprintf(os.Stderr, "[IRIS] failed to start shell: %v\n", err)
+		return
+	}
+	_ = tts.Close()
+
 	stdinFile := os.Stdin
 	if !term.IsTerminal(int(stdinFile.Fd())) {
 		if tty, ttyErr := os.OpenFile("/dev/tty", os.O_RDWR, 0); ttyErr == nil {