Feature: per-command user selection (--user flag) #13088

Describe the feature or problem you'd like to solve

When multiple accounts are authenticated via gh auth login, the only way to use a specific account is to switch globally with gh auth switch. This is cumbersome when you frequently need to alternate between accounts (e.g., a personal account and an Enterprise Managed User) within the same terminal session.

Current workflow:

gh auth switch --user personal-account
gh issue create --repo myorg/repo ...
gh auth switch --user work-emu-account   # don't forget to switch back!

This is error-prone — it's easy to forget to switch back, causing subsequent commands to run as the wrong user.

Proposed solution

Add a global --user flag available on all gh commands that overrides the active account for that single invocation:

gh issue create --repo myorg/repo --user personal-account ...
gh pr list --repo corp/repo --user work-account

This would be especially useful for:

• Enterprise Managed Users who need to interact with both EMU-org repos and personal repos
• CI scripts that need deterministic account selection without global state mutation
• Shell aliases/functions that target specific accounts

How will it benefit CLI and its users?

• Eliminates the error-prone switch → command → switch back pattern
• Enables concurrent multi-account usage in scripts without race conditions
• No global state mutation — pure, predictable command behavior

Additional context

Related to #326 (multiple account credentials). This proposal is a narrower, more immediately actionable enhancement that works with the existing gh auth switch infrastructure — it just needs a way to override the active account per-invocation rather than only globally.

The GH_TOKEN env var partially addresses this but requires managing raw PATs. A --user flag would work with the credentials already stored by gh auth login.