The `+` menu agent launcher had no golden coverage in the Windows lane, so a
Windows-only break anywhere in its chain (detection row, startup-plan build,
tab create, PTY spawn, startup-command injection) could ship unnoticed.
Adds a golden spec that launches a stub agent from the menu and asserts the
agent's own banner reached the pane — a tab that spawned a bare shell instead
is indistinguishable at the store/tab layer. Runs two agents everywhere, and
on Windows also PowerShell, cmd, Git Bash and a WSL project runtime.
Jinjing
c170ac2317