test: add sanity test for skills installation

Add a sanity test that verifies the skills CLI installation works:
- Runs `npx skills add . -a claude-code -y` using current directory
- Checks command doesn't fail (exit code 0)
- Verifies .claude/skills directory is created
- Dynamically discovers skills in the repo and verifies they're installed
- Checks SKILL.md exists in each installed skill

The test runs on every push to main and pull requests.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -12,9 +12,12 @@
 		"lint": "biome lint --write .",
 		"lint:check": "biome lint .",
 		"check": "biome check --write .",
-		"ci:check": "biome ci ."
+		"ci:check": "biome ci .",
+		"test": "vitest run",
+		"test:sanity": "vitest run test/sanity.test.ts"
 	},
 	"devDependencies": {
-		"@biomejs/biome": "2.3.11"
+		"@biomejs/biome": "2.3.11",
+		"vitest": "^3.0.0"
 	}
 }