Document why vite-tsconfig-paths can't be replaced by resolve.tsconfigPaths

Vite >=8 warns the plugin is replaceable by the native experimental
resolve.tsconfigPaths option. It isn't for this repo: the native option
applies the nearest tsconfig.json's own paths per importing file, while
our paths map lives only in the root tsconfig — per-workspace tsconfigs
under packages/* and vendor/* have none, so native resolution falls
through to package.json exports (lib/, absent until yarn build) and
every unbuilt test import fails (verified on vite 8.0.16/vitest 4.1.8).

vitest.config.ts

@@ -2,6 +2,21 @@ import tsconfigPaths from 'vite-tsconfig-paths'
 import { defineConfig } from 'vitest/config'
 
 export default defineConfig({
+  // Vite ≥8 warns that this plugin can be replaced by the native (experimental)
+  // `resolve.tsconfigPaths: true`. It cannot — keep the plugin. Tests run
+  // unbuilt (see AGENTS.md): bare workspace names like `cordis` or
+  // `@deepseek-ai/dsh-llm` must resolve to src/, and the only place that
+  // mapping exists is the root tsconfig.json `paths` map inherited by
+  // tsconfig.test.json. The native option is a bare boolean: for each
+  // importing file it discovers the NEAREST tsconfig.json and applies that
+  // file's own `paths`. Every workspace under packages/* and vendor/* has its
+  // own tsconfig.json without `paths`, so native resolution maps nothing,
+  // falls through to package.json exports (lib/, absent until `yarn build`),
+  // and every test file fails to import (verified on vite 8.0.16 /
+  // vitest 4.1.8). Making it work would mean copying the paths map into all
+  // 15 workspace tsconfigs — including vendor/* ones, which are pinned
+  // upstream copies (vendor/README.md). The plugin's `projects` option
+  // instead applies the one root map to every importer.
   plugins: [tsconfigPaths({ projects: ['./tsconfig.test.json'] })],
   test: {
     include: ['packages/*/tests/**/*.spec.ts'],