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/context/workspace-context/README.md

@@ -42,7 +42,7 @@ These instructions apply to work under `packages/app`. Use them as guidance when
 
 A same-file edit starts with `Updated instructions from: <path>` and says to use the new content instead of the previously loaded content. A candidate switch additionally names the old path. When no candidate remains, the message is `Instructions removed: <path>` followed by `The previously loaded instructions from this file no longer apply.` Literal `</system-reminder>` text inside an instruction file is escaped so file content cannot close the plugin-owned frame.
 
-The plugin owns the complete `<system-reminder>` framing, and every `context/message` (from this plugin or any other) reaches the model verbatim as a user-role message with no wrapping.
+The core `context/message` envelope is disabled for these messages because the plugin already owns the complete `<system-reminder>` framing. This is caller-selected with `envelope: 'raw'`; ordinary injected context still receives the canonical `<context source="...">` envelope.
 
 ## State And Refresh