Retire the readline front door and the repl-agent example

Delete packages/ui/stdio and examples/repl-agent; rename stdio-demo to
@deepseek-ai/dsh-tui-demo (TUI-only, refuses pipes before Loader boot).
tui-agent owns the coding composition inline; echo-agent and the CI demo
smoke move to the one-shot cli-demo bin, which gains -p/--prompt. The
UI-independent with-key e2es move verbatim to tui-agent. SDK wizard's
'stdio' interface becomes 'tui'. PTY testing stays confined to TUI
surfaces; all other subprocess tests ride pipes.

See .agents/notes/implemented/simplification/2026-07-20-retire-readline-front-door.md

eslint.config.mjs

@@ -14,7 +14,6 @@ export default tseslint.config(
       '**/.doc-typecheck-*/**',
       'website/.generated/**',
       'vendor/**', // vendored source keeps upstream style and idioms
-      'native/**', // imported landlock-run subtree: self-contained workspace with its own gates (native/README.md)
       '**/*.js',
       '**/*.mjs',
       '*.config.ts', // root tool configs (vitest, tsdown) — no project service