refactor: initial build system refactor for unified skills

- Renamed postgres-best-practices-build to skills-build
- Moved postgres files to src/postgres/ subdirectory
- Updated BUILD_DIR paths in postgres/config.ts
- Updated type imports to reference ../types.js
- Created IMPLEMENTATION_GUIDE.md with detailed step-by-step tasks

This is the foundation for a unified build system that will support
both postgres-best-practices (rule-based) and supabase (reference-based) skills.

Next steps documented in IMPLEMENTATION_GUIDE.md (Tasks 1-6).

packages/skills-build/package.json

@@ -0,0 +1,18 @@
+{
+	"name": "postgres-best-practices-build",
+	"version": "1.0.0",
+	"type": "module",
+	"author": "Supabase",
+	"license": "MIT",
+	"description": "Build system for Supabase agent skills",
+	"scripts": {
+		"build": "tsx src/build.ts",
+		"validate": "tsx src/validate.ts",
+		"dev": "npm run validate && npm run build"
+	},
+	"devDependencies": {
+		"@types/node": "^20.10.0",
+		"tsx": "^4.7.0",
+		"typescript": "^5.3.0"
+	}
+}