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/echo-agent/tests/echo.e2e.ts

@@ -13,7 +13,7 @@ import { afterEach, describe, expect, it } from 'vitest'
  *
  * This is the guard the per-file unit suite structurally cannot be: it drives
  * the `@deepseek-ai/dsh-stdio-agent` app plugin, the `@deepseek-ai/dsh-agent-core`
- * bundle it loads, the extracted `@deepseek-ai/dsh-ui-stdio` plugin, AND the
+ * bundle it loads, the app's in-package readline UI module, AND the
  * example-local `mock-llm.ts` / `echo-tool.ts` through their REAL load path, so
  * a broken plugin export shape (a stray `export default` that `unwrapExports`
  * would collapse, dropping `inject`/`Config`) fails here even though hand-mounted