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).

tsconfig.json

@@ -53,7 +53,6 @@
     { "path": "./packages/ui/acp" },
     { "path": "./packages/ui/acp-agent" },
     { "path": "./packages/ui/stdio-agent" },
-    { "path": "./packages/support/ui-stdio" },
     { "path": "./packages/support/llm-replay" },
     { "path": "./packages/subagent/subagent" },
     { "path": "./packages/support/subagent-mock" },