feat: fold the Code Mode demos into demo:code-mode with a UI argument

Code Mode is the point; the UI is just the surface it happens to wear.
demo:code and demo:acp-code collapse into one dispatcher
(scripts/demo-code-mode.mjs): `pnpm run demo:code-mode [repl|acp]` —
repl (default) boots the stdio REPL over examples/code-agent, acp
serves examples/acp-agent's code-mode overlay; each UI runs the exact
node invocation its standalone script ran, and an unknown argument
fails loud with usage. All nine references across READMEs, the RFC,
the overlay header, and the keyless-smoke comment renamed. Smoked all
three paths: usage exit 2, ACP initialize handshake, REPL boot + EOF.

examples/code-agent/tests/keyless-smoke.e2e.ts

@@ -42,7 +42,7 @@ async function bootAndEof(): Promise<{ stdout: string; code: number }> {
   return new Promise((resolve, reject) => {
     const proc = spawn(
       process.execPath,
-      // --expose-internals: cordis.yml loads the HMR plugin (mirrors demo:code).
+      // --expose-internals: cordis.yml loads the HMR plugin (mirrors demo:code-mode).
       ['--expose-internals', '--import', tsxLoader, binScript, configPath],
       {
         cwd,