An edge sign-in for someone with no forge account creates nothing and goes to the forge's own signup #2593

Part 1 of #668, the account rule alone. Today ensureEdgeUser in src/core/users.ts makes an account on first sight.

Done looks like:

• [ ] ensureEdgeUser only finds: an existing active user by name is returned, and an unknown login returns undefined and creates nothing.
• [ ] The request is then treated as signed out, and the page offers /signup, which applies the signup setting and its checks as for anyone. With signup closed, the person reads that signup is closed. They are never let in.
• [ ] Accounts the edge already made keep signing in as before.
• [ ] Tests: an unknown edge login creates no user row, an existing edge-made account still signs in, and with signup closed an unknown edge login is refused. Keep the existing passport and edge tests green, and name any unrelated red in the pull rather than chase it.

Parked by the crew: the check stayed red through 2 automatic tries (pull #2599 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Parked by the crew: the check failed ("every caller of createUser says where its account comes from") and the decider saw no point in another try (pull #2762 closed).

Parked twice since a person last spoke here, so it is the planner's now. Every reason:

1. the check stayed red through 2 automatic tries (pull #2599 closed)
2. the check failed ("every caller of createUser says where its account comes from") and the decider saw no point in another try (pull #2762 closed)

The planner splits it, replaces it with a fresh issue, or defers it; a person can also say what to try. Nothing starts here until then, and the board lists it.

State: replaced by #2763. Both pulls were stopped by test/signup-gate-parent.test.ts, which lists the edge as a way to make accounts. The fresh issue says to remove that entry on purpose, because closing that door is the point.