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/examples/cli-demo/tests/cli.spec.ts

@@ -162,15 +162,19 @@ describe('parseCliArgs', () => {
       kind: 'run', configPath: 'custom.yml', outputFormat: 'stream-json', task: 'do it',
     })
     expect(parseCliArgs(['--', '-task'])).toMatchObject({ task: '-task' })
+    expect(parseCliArgs(['-p', 'flag task'])).toMatchObject({ task: 'flag task' })
+    expect(parseCliArgs(['--prompt', 'long-flag task'])).toMatchObject({ task: 'long-flag task' })
     expect(parseCliArgs(['--help', 'ignored'])).toEqual({ kind: 'help' })
   })
 
   it('rejects missing, blank, extra, invalid-format, and unsupported flags', () => {
     expect(() => parseCliArgs([])).toThrow('received 0')
     expect(() => parseCliArgs(['   '])).toThrow('must not be blank')
+    expect(() => parseCliArgs(['-p', '   '])).toThrow('must not be blank')
     expect(() => parseCliArgs(['one', 'two'])).toThrow('received 2')
+    expect(() => parseCliArgs(['-p', 'task', 'positional'])).toThrow('mutually exclusive')
     expect(() => parseCliArgs(['--output-format', 'xml', 'task'])).toThrow('unsupported output format')
-    expect(() => parseCliArgs(['-p', 'task'])).toThrow('Unknown option')
+    expect(() => parseCliArgs(['-x', 'task'])).toThrow('Unknown option')
   })
 })