Retire multi-account migration - #14430

Description

When initial multi account support was added to gh in v2.40.0 (December 2023, so almost 3 years ago at this point), we had to perform some data migrations. The code for this, with lots of nice explanations can be found in multi-account.go, and it has been running on every invocation of gh for 3 years:

The TL;DR was that we needed to take host level user/token data and namespace it by username. For example, a hosts entry of:

//	github.localhost:
//	  user: monalisa
//	  git_protocol: https
//    oauth_token: xyz

would become:

// github.localhost:
//	 user: monalisa
//	 git_protocol: https
//   oauth_token: xyz
//   users:
//	   monalisa:
//	     oauth_token: xyz

A similar operation would occur for the keyring with a key of:

"gh:" + hostname

being copied to:

"gh:" + hostname + ":" + username

This set the stage for gh auth switch to pick the user and token from a list and "activate" them in the config/keyring entry. The data format was both backwards and forwards compatible, to allow users to move back and forward between versions without breaking anything. However, this global keyring entry has been the source of a number of bugs such as:

• gh auth token --secure-storage ignores the config's active account and returns the globally-active keyring token #14370
• Keychain lookup ignores account field, breaks multi-account setups #12885

These bugs occur because people use GH_CONFIG_DIR to swap config files (and therefore active users). This can cause a split brain in some cases where the active user in the config, and the user owning the token in the keyring aren't the same. We can fix the code paths, but we're now 3 years on and I don't think we should give much concern to compatability pre v2.40.0, so I want to remove that keyring entry altogether.

This PR doesn't do that work, but it does remove the migration that circles the area and just sits there as debt.

How did you test this change?

➜  williammartin-probable-robot git:(williammartin-fix-keyring-user-lookup) ./bin/gh version       
gh version 2.100.0-103-g5fe0f89e1 (2026-09-11)
https://github.com/cli/cli/releases/latest
➜  williammartin-probable-robot git:(williammartin-fix-keyring-user-lookup) ./bin/gh api /zen | cat
Encourage flow.% 

Authorship and follow-up

Who wrote this:

• A human wrote it.
• An agent wrote it under close human direction.
• An agent wrote it independently, and no human has guided the implementation beyond the initial prompt.

Who answers review comments:

• @williammartin will read and reply directly. Name the account.
• An agent will draft replies and @username will read them before they are posted.
• Nobody has explicitly committed to replying.