open-platform.sh sign-in is a linked method, not a back door: it never creates an account outside the signup rules #668

A first sign-in through the platform edge creates a forge account on the spot, outside the forge's own signup rules. That is a second way in. There must be one way in, the forge's own. The open-platform.sh (Forgejo) sign-in becomes a method a person links to an account they already have, kept through the migration and removable later.

Acceptance criteria

• [ ] An edge or Forgejo sign-in for a person with no linked forge account does not create one. It goes to the forge's own signup, which applies the signup setting (#624), the human check and rate limits (#661), email verification (#659), quotas (#657) and the zero starting budget (#656).
• [ ] A signed-in person can link and unlink their open-platform.sh identity from account settings. After that, the linked sign-in reaches the same account and session.
• [ ] Accounts the edge already made (created before this lands) are kept and listed for the owner, with their linked method shown.
• [ ] The forge works with linked methods turned off entirely. A setting turns each one off.
• [ ] Tests: an edge sign-in for an unknown person creates nothing; linking then signing in reaches the linked account; with signup closed, an unknown edge sign-in is refused.

This can land before #666. The account-creation rule (an unknown edge sign-in creates nothing and goes to the forge signup instead) needs only #624, and it blocks reopening (#663). Linking through the full authorization server follows #666.

Parked by the crew: the check failed and the decider saw no point in another try (pull #744 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 stayed red through 2 automatic tries (pull #2581 closed).

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

1. the check failed and the decider saw no point in another try (pull #744 closed)
2. the check stayed red through 2 automatic tries (pull #2581 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: split into #2593, #2594, #2595 and #2596. Two tries at the whole ask went red (pulls #744 and #2581). What the fresh ones do differently: one rule per run. #2593 is only the account rule: an unknown edge sign-in creates nothing and goes to the forge's own signup. #2594 lists the accounts the edge already made. #2595 is the setting that turns the edge off. #2596 is link and unlink in account settings. #663 and #669, which waited on this, now wait on #2593.