refactor(ui): fold the stdio UI helper into the stdio app

The readline UI lives inside @deepseek-ai/dsh-stdio-agent as the
in-package stdio-chat module; the packages/support/ui-stdio package is
gone. The app's front-door cluster always includes this UI and nothing
else composes it, so the boundary bought manifest/tsconfig/module-graph/
README/publint surface for a helper that is not independently
swappable — and a product app no longer depends on a support package
documented as not-product-surface.

createStdioChat, the StdioRuntime test seam, and both unit suites moved
verbatim (imports rewired to the module path); the named
name/inject/Config/apply export shape stays, being the contract the
app's ctx.plugin mount consumes. Coverage stays per-file 100%; the
built-bin smoke under plain node and both keyless Loader-path smokes
prove the published artifact and the demos end-to-end.

Implements docs/rfc/implemented/simplification/2026-07-04-fold-stdio-ui-helper.md
(moved from proposed/ and amended to the shipped shape).

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

@@ -9,8 +9,8 @@ import { afterEach, describe, expect, it } from 'vitest'
  * Keyless Loader-path smoke for examples/coding-agent: boot the REAL example
  * through the `@deepseek-ai/dsh-stdio-agent` bin against its `cordis.yml` (the
  * cordis Loader, `unwrapExports`, the full plugin tree incl. the
- * `@deepseek-ai/dsh-agent-core` bundle and the extracted
- * `@deepseek-ai/dsh-ui-stdio`), then close stdin with no prompt and assert the
+ * `@deepseek-ai/dsh-agent-core` bundle and the app's in-package readline UI
+ * module), then close stdin with no prompt and assert the
  * ready banner + a clean exit.
  *
  * No prompt is ever sent, so the model is NEVER called — this is why it runs