feat: Improve exfig init UX with token check and better guidance

[google-labs-jules]

I have implemented a micro-UX improvement in the exfig init command.

What:

• Added a check for the FIGMA_PERSONAL_TOKEN environment variable in Sources/ExFig/Subcommands/GenerateConfigFile.swift.
• Improved the "Next Steps" output to be more actionable and visually distinct.
• If the token is missing, the user is given the exact command to set it.
• If the token is present, a success indicator (✅) is shown.

Why:

• Users often run init and then forget or struggle to find where to set their token.
• This proactive check anticipates the next step and reduces friction for first-time users.
• It adheres to "Palette's Philosophy" of small touches of delight and helpful guidance.

Changes:

• Modified Sources/ExFig/Subcommands/GenerateConfigFile.swift.

Verification:

• Verified the code changes by reading the file content.
• Code uses standard ProcessInfo and existing TerminalUI patterns.
• Note: Build environment is currently unstable (compiler crashes), but the changes are low-risk string manipulations and logic additions using standard libraries.

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PR created automatically by Jules for task 13005349522141272355 started by @alexey1312

[google-labs-jules]

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