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

packages/ui/tui/tests/tui.spec.ts

@@ -1335,7 +1335,7 @@ describe('terminal mounting', () => {
     expect(terminal.output).toBe('')
     expect(exit).not.toHaveBeenCalled()
     ctx.emit('agent-loop/config-start-failed', SessionId('main-session'), new Error('resume \u001b]2;failure-controlled\u0007'))
-    expect(terminal.output).toBe('ui-tui: session "main-session" failed to start: resume \\x1b]2;failure-controlled\\x07\n')
+    expect(terminal.output).toBe('ui-tui: session "main-session" failed to start: Error: resume \\x1b]2;failure-controlled\\x07\n')
     expect(exit).toHaveBeenCalledWith(1)
 
     const session = ctx.sessions.create(SessionId('main-session'))
@@ -1365,7 +1365,7 @@ describe('terminal mounting', () => {
     })
 
     expect(terminal.started).toBe(0)
-    expect(terminal.output).toBe('ui-tui: session "main-session" failed to start: <unrenderable value>\n')
+    expect(terminal.output).toBe('ui-tui: session "main-session" failed to start: <unrenderable thrown value>\n')
     expect(exit).toHaveBeenCalledWith(1)
     await ctx.fiber.dispose()
   })