supporting transparent hex colors

[tototofu123]

add support for 4-digit and 8-digit HEX color codes (e.g., #RGBA and #RRGGBBAA) in the design.md CLI linter. Previously, the linter only accepted opaque 3 or 6-digit hex codes.

Related Issues
Fixes #14

Changes

• Type Definition: Added an optional a (alpha) channel field to the ResolvedColor interface in spec.ts.
• Validation: Updated the isValidColor regex to accept 4 and 8 character hex strings.
• Parsing Logic:
  • Updated parseColor in handler.ts to correctly normalize #RGBA shorthand to #RRGGBBAA.
  • Implemented alpha extraction logic (converting 0-255 hex values to a 0-1 float).
  • Exported parseColor for better testability.
• Tests: Added new test cases in handler.test.ts to verify normalization and alpha extraction for transparent hex codes.

Verification

• Ran bun test in packages/cli.
• Verified that all 72 existing and new tests pass.

[davideast]

@tototofu123 This is so great! Thank you for a well done PR.