fix(pty): treat background jobs as non-interactive to avoid SIGTTOU stop (#324)

* fix(pty): treat background jobs as non-interactive to avoid SIGTTOU stop

IsInteractiveTerminal only checked that stdin/stdout are TTYs. A background
job (pmg npm run test &) still has the TTY on stdin/stdout, so pmg picked
PTY mode and called tcsetattr to enter raw mode. Changing terminal modes
from a background process group makes the kernel stop the process with
SIGTTOU, leaving the job hanging in Stopped state.

Check that the process group is the terminal's foreground process group
(tcgetpgrp == getpgrp) before treating the terminal as interactive, so
background jobs fall through to direct execution.

Fixes #322

https://claude.ai/code/session_01PBBo5CKkzg68MMGrgCTQcY

* test(pty): fail on output copy timeout to avoid racy buffer read

Reading the output buffer after a silent select timeout races with the
io.Copy goroutine still writing to it. Fail the test on timeout instead.

Also fix a grammar nit in the IsInteractiveTerminal doc comment.

https://claude.ai/code/session_01PBBo5CKkzg68MMGrgCTQcY

---------

Co-authored-by: Claude <noreply@anthropic.com>

internal/pty/foreground_unix.go

@@ -0,0 +1,21 @@
+//go:build !windows
+
+package pty
+
+import (
+	"golang.org/x/sys/unix"
+)
+
+// isForegroundProcess reports whether this process belongs to the terminal's
+// foreground process group. A background job (e.g. `pmg npm install &`) still
+// has the TTY on stdin/stdout, but changing terminal modes or reading from
+// the TTY in a background process group triggers SIGTTOU/SIGTTIN, which stops
+// the process.
+func isForegroundProcess(fd uintptr) bool {
+	foregroundPgrp, err := unix.IoctlGetInt(int(fd), unix.TIOCGPGRP)
+	if err != nil {
+		return false
+	}
+
+	return foregroundPgrp == unix.Getpgrp()
+}