refactor: unify the Code Mode demos on base-plus-overlay

Both demo:code-mode UIs now share one mechanism: the base example plus a
same-shaped code-mode.cordis.yml include overlay (insert the worker
runtime, flip tools.mode). Previously the REPL side was a hand-forked
example (examples/code-agent) that had also silently diverged — it
dropped compaction and the subagent stack — so the demo's UI argument
switched agents, not just surfaces. The fork is retired: coding-agent
gains the overlay, a Code Mode README section absorbing code-agent's,
and both of its tests (the keyless boot guard, retargeted at the
overlay; the with-key RFC proof, which hand-mounts its own harness and
moves untouched). The RFC's composed-surface and e2e-tier lines, the
examples index, the AGENTS.md smoke table, and the dsh-tools README
link now describe the overlay shape.

Verified live: overlay keyless smoke, with-key code-mode e2e from its
new home, demo:code-mode banner + EOF exit, and the acp handshake.

scripts/demo-code-mode.mjs

@@ -1,18 +1,20 @@
 /**
  * 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.
+ * point — the UI is just the surface it happens to wear: each UI boots its
+ * base example through that example's `code-mode.cordis.yml` overlay
+ * (include ./cordis.yml, flip `tools.mode` to `code`, insert the
+ * worker-thread code runtime). 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).
+// Each UI's node invocation, verbatim what its base demo script runs plus
+// the overlay config (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']],
+  ['repl', ['--expose-internals', '--import', 'tsx', 'packages/ui/stdio-agent/src/bin.ts', 'examples/coding-agent/code-mode.cordis.yml']],
   ['acp', ['--import', 'tsx', 'packages/ui/acp-agent/src/bin.ts', 'examples/acp-agent/code-mode.cordis.yml']],
 ])