fix(build): pin tsdown to the native config loader

Root build scripts, the desktop bundle and the dev:web watcher pass the
native loader so tsdown never selects the undeclared unrun fallback. A spec
covers the build.ts rejection path.

docs/development.md

@@ -8,7 +8,8 @@ The setup tutorial takes a new contributor from prerequisites to a checked check
 
 ### Prerequisites
 
-- Node.js supports 22.19+ and 24+. CI covers 22.19, 24, and 26; see the [Node engine floor Agent Note](../.agents/notes/implemented/process/2026-07-06-node-engine-floor.md). The build loads `tsdown.config.ts` through Node type stripping, so `pnpm run build` rejects a Node.js process where `process.features.typescript` is false, such as one with `--no-experimental-strip-types` in `NODE_OPTIONS`.
+- Node.js supports 22.19+ and 24+. CI covers 22.19, 24, and 26; see the [Node engine floor Agent Note](../.agents/notes/implemented/process/2026-07-06-node-engine-floor.md).
+- Node.js TypeScript type stripping enabled. The repository build scripts load `tsdown.config.ts` with tsdown's native config loader, so they fail when `--no-experimental-strip-types` is in `NODE_OPTIONS` or the Node.js build lacks TypeScript support; `pnpm run build` checks this first and names the cause.
 - Corepack-enabled pnpm. The repo pins `[email protected]` in `package.json`; run `corepack enable` if `pnpm --version` does not resolve through Corepack.
 - Git 2.26 or newer; hook setup enables Git's worktree-specific configuration extension.
 - Optional: a DeepSeek API key for the Web, headless, and ACP automation demos and real-API e2e tests.