feat(Claude extension): added Claude extension

[gastoner]

Adds in-built claude extension that registeres skills from claude/skills folder

Closes #1047

You can test this by creating some skill in claude/skills folder
Then enable extension (the skill should be visible in the list of skills)
Disable extension, all registered skills should now be unregistered (not visible)

[codecov]

Codecov Report

❌ Patch coverage is 28.00000% with 36 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
extensions/claude/scripts/build.js	0.00%	18 Missing ⚠️
...ages/main/src/plugin/extension/extension-loader.ts	18.18%	9 Missing ⚠️
extensions/claude/src/extension.ts	0.00%	7 Missing ⚠️
extensions/claude/src/claude-extension.ts	87.50%	1 Missing ⚠️
packages/main/src/plugin/skill/skill-manager.ts	83.33%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!