chore: close quality-gate coverage gaps (#1689) (#1766)

Closes #1689. Typecheck cli/tui src, add a core/ format+lint gate, and gate core/json + core/client coverage. See PR #1766 for the full review history.

eslint.config.js

@@ -0,0 +1,38 @@
+import js from "@eslint/js";
+import globals from "globals";
+import tseslint from "typescript-eslint";
+import { defineConfig, globalIgnores } from "eslint/config";
+
+// Root-level lint gate for the shared `core/` package. Each client's own
+// `eslint .` is scoped to its own directory, so nothing reached `core/` before
+// (#1689) — this config closes that gap. `core/` is isomorphic TypeScript
+// (browser-side OAuth + Node backends + shared runtime), so both browser and
+// Node globals apply; there is no JSX in `core/`, so no React plugin is needed.
+export default defineConfig([
+  globalIgnores(["core/**/build/**", "core/**/dist/**"]),
+  {
+    files: ["core/**/*.{ts,tsx}"],
+    extends: [js.configs.recommended, tseslint.configs.recommended],
+    languageOptions: {
+      ecmaVersion: 2022,
+      sourceType: "module",
+      globals: {
+        ...globals.node,
+        ...globals.browser,
+      },
+    },
+    rules: {
+      // An `_`-prefix is `core/`'s explicit "intentionally unused" marker —
+      // interface-conformance params in fakes, destructuring-rest omissions,
+      // and reserved-for-later args. Honor it rather than deleting signal.
+      "@typescript-eslint/no-unused-vars": [
+        "error",
+        {
+          argsIgnorePattern: "^_",
+          varsIgnorePattern: "^_",
+          caughtErrorsIgnorePattern: "^_",
+        },
+      ],
+    },
+  },
+]);