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/tsconfig.json

@@ -0,0 +1,16 @@
+{
+	"compilerOptions": {
+		"target": "ES2022",
+		"module": "ESNext",
+		"moduleResolution": "bundler",
+		"esModuleInterop": true,
+		"strict": true,
+		"skipLibCheck": true,
+		"outDir": "dist",
+		"rootDir": "src",
+		"declaration": true,
+		"resolveJsonModule": true
+	},
+	"include": ["src/**/*"],
+	"exclude": ["node_modules", "dist"]
+}