build: add build:core to compile the app-consumable core to dist/ (Phase 5d-2)

tsconfig.build.json emits providers/* + persist (+ their parsing.mjs/types deps)
to dist/ as ESM JS + .d.ts. Output is verified node:sqlite-free and functionally
runnable (parse+persist against an injected db). Foundation for the app importing
the shared core.
> obelisk@0.1.0 build:core
> rm -rf dist && tsc -p tsconfig.build.json; dist/ is gitignored.

package.json

@@ -8,7 +8,8 @@
   "scripts": {
     "test": "node --test tests/*.test.mjs",
     "typecheck": "tsc --noEmit",
-    "lint": "eslint ."
+    "lint": "eslint .",
+    "build:core": "rm -rf dist && tsc -p tsconfig.build.json"
   },
   "devDependencies": {
     "@eslint/js": "^10.0.1",