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).

AGENTS.md

@@ -114,8 +114,6 @@ packages/    Harness packages, grouped by role at packages/<group>/<pkg>/.
                     acp bridge, NO stdout logger + a bin (the demo:acp front door)
   support/        dev/test/example infrastructure (lower compat expectations)
     invariants/     dev-mode event-contract invariants + session-log freeze
-    ui-stdio/       minimal stdio (readline) UI plugin: renders agent/* events,
-                    feeds stdin lines to the agent (shared by the demos)
     llm-replay/     record/replay adapter: short-circuits llm/stream from a
                     recorded session JSONL (keyless snapshot tests)
     subagent-mock/  scripted SubagentProvider for deterministic seam/tool tests