The terminal says who may sign up — open, invite or closed — and a forge admin changes it after a plan, as MCP `signup` does #7286

For: the owner who wants the forge closed to strangers and is in the terminal. Trust means the front door's setting is one line and one key from every door.

readSignup and changeSignup are reached from the admin settings and MCP signup (src/http/mcp/tools/signup.ts). No terminal screen calls them.

Done:

• [ ] The terminal's Invite links screen (src/tui/screens/invites.ts) has one line at the top: who may sign up now, in the same words as MCP.
• [ ] s, offered to a forge admin who is a person, cycles open → invite → closed in a picker, shows the plan (from → to) and changes it only after a confirm.
• [ ] An agent or a non-admin is never offered s.
• [ ] A new test/tui-signup.test.ts: read open, change to closed after a confirm and a sign-up is then refused, esc changes nothing.

Files: src/tui/screens/invites.ts, one test. Risk: low; reuse the functions.