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

examples/tui-agent/code-mode.cordis.yml

@@ -9,9 +9,10 @@
         name: '@deepseek-ai/dsh-tui-demo'
         config:
           provider: deepseek
-          model: deepseek-v4-flash
+          model: deepseek-v4-pro
           resumeSessionId: !!js process.env.RESUME_SESSION_ID
           persistenceRoot: './.sessions'
+          resumeCommand: 'dsh --resume {session}'
           workspaceContext:
             maxBytes: 65536
           tools: