Support rill sudo user open <email> + handle repeated assumes gracefully

[begelundmuller]

Changes:

• Adds optional support for passing an email directly to rill sudo user open so you don't have to run rill sudo user assume separately
• Prints the open link when running rill sudo user open in case you want to open in a non-standard browser
• Adds automatic handling for when assuming a user while already assumed as another user

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!