polish(claude): Prefix FXA skills with fxa-

[dschom]

Because

• We want to be consistent
• We want to make fxa specific skills obvious

This pull request

• Prefixes skills in .claude/skills with fxa-

Issue that this pull request solves

Closes: (issue number)

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).
• I have manually reviewed all AI generated code.

Screenshots (Optional)

Please attach the screenshots of the changes made in case of change in user interface.

Other information (Optional)

Any other information that is important to this pull request.