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/acp-agent/code-mode.cordis.yml

@@ -3,7 +3,7 @@
 # (the registry offers exactly one wire tool, run_code, plus the generated
 # TypeScript SDK prompt section) and the worker-thread code runtime joins the
 # tree as `ctx.codeRuntime`. The dsh-acp-agent bin boots this file for
-# `pnpm run demo:acp-code` and when the snapshot harness records the
+# `pnpm run demo:code-mode acp` and when the snapshot harness records the
 # code-mode scenarios; DSH_SNAPSHOT=replay swaps it for the sibling
 # code-mode.cordis.snapshot.yml. A config patch REPLACES the entry's whole
 # config, so the base entry's fields are restated verbatim.