docs(cli): document account switching

[c8dhjp4tyv-bit]

Recreated on the rewritten main after #1104 was auto-closed during repository maintenance. This carries the same reviewed change set on the new history.

Summary

• document how to switch the account used by the CLI
• mention the existing /logout and /signout commands
• point users to the freebuff login browser flow after logout

Fixes #1086

Validation

Prior validation before the history rewrite:

• verified command-registry.ts defines /logout with /signout as an alias
• verified login-modal.tsx prompts Press ENTER to login... before starting the browser login flow
• verified cli-args.ts accepts freebuff login
• git diff --check passed

[codebuff-team]

Thanks for the resubmission after the history rewrite. This is a small, focused documentation change that adds a genuinely missing piece: how to switch accounts.

The claims are verifiable against the codebase (/logout with /signout alias in command-registry.ts, the login prompt in login-modal.tsx, and freebuff login in cli-args.ts), and the wording is accurate and concise. It sits in README.md, which is in scope for the mirror.

Nothing blocking here — it's a low-risk, useful addition. One minor nit: consider clarifying that freebuff login is meant to be run from a fresh shell/session (not the same interactive prompt as pressing Enter), since a new contributor reading only this section might wonder if the two options are redundant or different flows. Not a blocker, just a clarity improvement a maintainer might request.

[c8dhjp4tyv-bit]

Clarified the account-switching wording as suggested: pressing Enter refers to signing in again in the same interactive session, while freebuff login is now explicitly described as being run from a fresh shell/session.