User-defined custom roles via roles.json

[lewiswigmore]

Currently Cicada ships with 4 built-in roles (Coder, Reviewer, Tester, Researcher).

Users should be able to easily add their own roles by editing
oles.json\ — this already works technically, but we should:

• Document how to add custom roles in README/USAGE.md
• Add example custom roles (e.g. \�rchitect, \devops, \pm)
• Validate custom role definitions on load with helpful error messages

Good first issue

This is mostly documentation + light validation work.