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.

scripts/demo-code-mode.mjs

@@ -0,0 +1,27 @@
+/**
+ * Boot the Code Mode demo under the UI named on the command line:
+ * `pnpm run demo:code-mode [repl|acp]`, default `repl`. Code Mode is the
+ * point — the UI is just the surface it happens to wear: `repl` starts the
+ * stdio REPL over examples/code-agent, `acp` starts the ACP server over
+ * examples/acp-agent's code-mode overlay. Both need DEEPSEEK_API_KEY
+ * (repo-root .env works). Anything else on the command line is a
+ * misconfiguration and fails loud with usage.
+ */
+import { spawn } from 'node:child_process'
+
+// Each UI's node invocation, verbatim what its standalone demo script ran
+// (the stdio bin keeps --expose-internals for the cordis Loader's HMR path).
+const UIS = new Map([
+  ['repl', ['--expose-internals', '--import', 'tsx', 'packages/ui/stdio-agent/src/bin.ts', 'examples/code-agent/cordis.yml']],
+  ['acp', ['--import', 'tsx', 'packages/ui/acp-agent/src/bin.ts', 'examples/acp-agent/code-mode.cordis.yml']],
+])
+
+const ui = process.argv[2] ?? 'repl'
+const args = UIS.get(ui)
+if (!args || process.argv.length > 3) {
+  console.error('usage: pnpm run demo:code-mode [repl|acp]')
+  process.exit(2)
+}
+
+const child = spawn(process.execPath, args, { stdio: 'inherit' })
+child.on('exit', (code, signal) => { process.exit(signal !== null ? 1 : code ?? 1) })