Reaching another account requires switching the active session

[scotwells]

Summary

Every datumctl command runs as the active session, so reaching another account or environment means running datumctl auth switch first and switching back afterwards. Scripts, CI jobs and agents that touch production and staging in one run have to change shared state between commands, and anything running at the same time sees the switch too.

Scope already has per-command overrides through flags and environment variables, but the session does not.

Acceptance criteria

• A user can run one command as a named session, with a flag or an environment variable, and the active session stays unchanged
• The command uses that session's own organization and project unless the user names a scope
• Naming an unknown or ambiguous session fails with a message that lists the valid choices
• Plugins run with a session override act as that session