feat(ui): add dedicated TUI package

Move the interactive pi-tui front door into @deepseek-ai/dsh-tui while keeping @deepseek-ai/dsh-stdio line-oriented for pipes. Select the terminal package in the demo app, preserve logger ownership, and cover the production Loader composition with a PTY smoke test.

examples/echo-agent/README.md

@@ -4,7 +4,7 @@ Runnable demo: stdin chat with a scripted mock model and an echo tool. The all-m
 
 ## What it shows
 
-This example is just a leaf `cordis.yml`: it loads the [`@deepseek-ai/dsh-stdio-demo`](../../packages/examples/stdio-demo) app (which bundles the whole [`@deepseek-ai/dsh-agent-spine-demo`](../../packages/examples/agent-spine-demo) spine, the console logger, JSONL persistence, the readline UI, and a pre-created `main` agent), and swaps in two example-local backends plus `hmr`:
+This example is just a leaf `cordis.yml`: it loads the [`@deepseek-ai/dsh-stdio-demo`](../../packages/examples/stdio-demo) app (which bundles the whole [`@deepseek-ai/dsh-agent-spine-demo`](../../packages/examples/agent-spine-demo) spine, JSONL persistence, the TTY-selected `dsh-tui`/`dsh-stdio` front doors, and a pre-created `main` agent), and swaps in two example-local backends plus `hmr`:
 
 - `mock-llm.ts` — a mock `LlmAdapter` that streams scripted responses and calls the `echo` tool when the user types "echo <something>". Registered with `ctx.llm.registerAdapter(['mock-echo'], …)`.
 - `echo-tool.ts` — a tool registered via `ctx.tools.register(defineTool(…))` with typed `execute` args; echoes text back uppercased.

examples/echo-agent/composition.md

@@ -20,7 +20,7 @@ flowchart LR
   cfg --> plugin_echo_stdio_agent
   plugin_echo_stdio_agent --> bundle_agent_core["@deepseek-ai/dsh-agent-spine-demo"]
   plugin_echo_stdio_agent --> bundle_jsonl["@deepseek-ai/dsh-session-persistence-jsonl"]
-  plugin_echo_stdio_agent --> frontdoor_stdio["readline UI<br/>console logger<br/>pre-created main agent"]
+  plugin_echo_stdio_agent --> frontdoor_stdio["dsh-tui (TTY) / dsh-stdio (pipes)<br/>pre-created main agent"]
   bundle_agent_core --> spine_llm["ctx.llm"]
   bundle_agent_core --> spine_sessions["ctx.sessions"]
   bundle_agent_core --> spine_tools["ctx.tools + tool-bash"]

examples/echo-agent/cordis.yml

@@ -22,7 +22,7 @@
 - id: bash
   name: '@deepseek-ai/dsh-bash-local'
 
-# The app pre-creates `main` on the mock model and supplies logging, persistence, and readline UI.
+# The app pre-creates `main` on the mock model and supplies persistence plus TTY-selected `dsh-tui`/`dsh-stdio` front doors.
 - id: stdio-agent
   name: '@deepseek-ai/dsh-stdio-demo'
   config: