Add Hex to canvas skill

[okoshy]

Summary

• Add a hex-to-canvas skill under the default skills/<skill-name>/SKILL.md plugin directory
• Document how to prompt Hex Threads for exact KPI values before building Cursor canvases
• Sync with the latest main branch, including the current Hex skill structure
• Update the README to list all current skills and include an example canvas workflow

Verification

• Ran git diff --check
• Confirmed skill frontmatter includes required name and description fields
• Confirmed the skill follows the current skills/<skill-name>/SKILL.md structure
• Confirmed no hex-thread-to-canvas references remain