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).

docs/rfc/implemented/simplification/2026-07-04-fold-stdio-ui-helper.md

@@ -0,0 +1,24 @@
+# RFC: Fold the stdio UI helper into the stdio app
+
+Status: implemented
+
+## Problem
+
+The readline UI was a whole package (`@deepseek-ai/dsh-ui-stdio` under `packages/support/`) whose only runtime importer was the app package `@deepseek-ai/dsh-stdio-agent`. The examples reach the readline UI by loading the app, never by composing the helper themselves; every other repo reference was mechanical or descriptive surface that existed BECAUSE the package boundary existed — manifest and tsconfig entries, generated module-graph rows, dependency-graph and README rows, and doc comments naming the package. The ui group README recorded the support placement rationale ("exists chiefly for the examples and the coverage gate — `ui/` is reserved for surfaces shipped as product"), which left a standing tension: a shipped product app depending on a support package documented as NOT product surface.
+
+The boundary bought package metadata, workspace and tsconfig references, module-graph rows, README entries, and publint surface for a helper that is not independently swappable: the stdio app's front-door cluster always includes the readline UI, and nothing else can meaningfully consume it.
+
+## Decision
+
+The helper lives inside `@deepseek-ai/dsh-stdio-agent` as the in-package `stdio-chat` module (`packages/ui/stdio-agent/src/stdio-chat.ts`): `createStdioChat`, its `StdioRuntime` test seam, and its unit tests (`packages/ui/stdio-agent/tests/stdio-chat.spec.ts`, `readline.spec.ts`) moved with it, so EOF handling, rendering, disposal, and piped-vs-TTY behavior stay unit-covered under the per-file coverage gate without hijacking process globals. The module keeps the named `name`/`inject`/`Config`/`apply` export shape — the contract the app's `ctx.plugin(uiStdio, …)` mount consumes — and the keyless Loader-path smokes in `examples/echo-agent` and `examples/coding-agent` keep guarding the app's export shape end-to-end.
+
+The `packages/support/ui-stdio` package is gone: manifest, tsconfig references, module-graph rows, and README rows deleted; the doc comments that named the package (the example e2e module docs, `packages/README.md`, the support and todo READMEs, [the ui group README](../../../../packages/ui/README.md)) describe the in-package module.
+
+## Why not promote it to `ui/` instead?
+
+Promotion would have resolved the support-vs-product mismatch while keeping the boundary — the right call only if the readline UI were an independently swappable integration or had a second composer, and the consumer census said neither. The structured ACP bridge stays its own package because it is the product protocol surface with its own contract and snapshot tiers; the readline helper is scaffolding for one app's front door. Re-extraction stays cheap pre-release: if a second product app wants the readline UI, split it back out then, with that consumer shaping the package contract.
+
+## Consequences
+
+- The stdio app owns its whole front door; a leaf `cordis.yml` still loads one app package and nothing changed shape for the demos.
+- A future standalone terminal UI that wants the helper as a package reintroduces it with that second consumer, rather than the repo keeping a boundary for hypothetical reuse.