build(skill): add build:skill for a readable, non-bundled skill artifact (Phase 6)

`npm run build:skill` emits dist/obelisk-skill/: the whole scripts/ Core
compiled to readable JS (module structure + comments preserved, ~1:1 with
source, no bundling/minifying per ADR-0004), plus schema.sql, SKILL.md,
references/, and a package.json. It excludes app/, tests/, and release/, which
is what keeps the artifact small.

- tsconfig.skill.json compiles scripts/**/*.{ts,mjs} (allowJs) with
  rewriteRelativeImportExtensions, so the .ts specifiers inside the .mjs shells
  (runtime.mjs -> ./core.js, indexer.mjs -> ./providers/*.js, ./persist.js) are
  rewritten to .js. declaration:false — the skill needs no .d.ts.
- packaging/skill-package.json provides the artifact's package.json; it sets
  "type":"module" because the emitted .ts->.js files are ESM.
- build:skill runs tsc then copies schema.sql/SKILL.md/references/package.json.

Add tests/build-skill.test.mjs: runs the real build:skill, asserts the artifact
structure, that no emitted .js/.mjs still imports a .ts module, and that the
compiled artifact builds an index and answers a search end-to-end under plain
Node (no type-stripping) against a temp HOME.

Verified: suite 123/123, typecheck clean, artifact smoke run green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tsconfig.skill.json

@@ -0,0 +1,11 @@
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "noEmit": false,
+    "outDir": "dist/obelisk-skill/scripts",
+    "rootDir": "scripts",
+    "declaration": false,
+    "rewriteRelativeImportExtensions": true
+  },
+  "include": ["scripts/**/*.ts", "scripts/**/*.mjs"]
+}