Mirrors `sse-session-cookie.ts` exactly. Mints short-lived 30-min HttpOnly
cookies for authenticating the Terminal-tab WebSocket upgrade against
the terminal-agent. Same TTL, same opportunistic-pruning shape, same
"scoped tokens never valid as root" invariant. Two registries instead of
one because the cookie names are different (`gstack_sse` vs `gstack_pty`)
and the token spaces must not overlap.
No callers yet — wired up in the next commit.
Garry Tan