refactor(examples): extract the app spine into dsh-agent-core + app packages

Implements docs/rfc/.../2026-06-20-extract-example-app-packages.md. Each
example was thick — a hand-rolled start.ts, an infra preamble, nested
base.yml/base-core.yml/acp-tail.yml includes, and a coupled front-door
cluster enforced only by prose. This moves the composition into packages so
each example is a thin leaf cordis.yml: pick the swappable backends, load one
app package.

New packages:
  - @deepseek-ai/dsh-agent-core (packages/core/agent-core): one bundle plugin
    that loads the providerless/executor-less/UI-less spine (timer + llm +
    sessions + system-prompt + tools + agents + invariants + tool-bash +
    agent-loop) via ctx.plugin(...) inside apply(), and forwards agent-loop's
    `agents` list as its own Config (export const Config = AgentLoop.Config,
    default []).
  - @deepseek-ai/dsh-stdio-agent (packages/ui/stdio-agent): terminal chat APP —
    agent-core + console logger + readline UI + a pre-created `main` agent, with
    a bin. The demo:echo/coding front door.
  - @deepseek-ai/dsh-acp-agent (packages/ui/acp-agent): ACP server APP —
    agent-core + JSONL persistence + the acp bridge, NO stdout logger, with a
    bin. The stdout-purity footgun is structurally unreachable from the leaf.

Amendment to the RFC: hmr stays a LEAF cordis.yml entry, not baked into
dsh-stdio-agent. hmr is a Loader-only dev plugin (throws without
--expose-internals; the in-process test tier can't even import its decorator
form), so a package statically importing it could never carry the per-file
coverage gate. Unlike the console logger, a stray hmr is not a stdout-purity
footgun, so leaving it at the leaf costs no safety. With hmr out, all three new
packages carry in-process unit specs at 100%.

Boot glue (Loader tail, .env load, snapshot-mode selection, stdin-dispose
lifecycle) moves into each app's bin; start.ts and base.yml/base-core.yml/
acp-tail.yml are deleted. Each app package gets a keyless real-load-path test
that boots through its bin + the cordis Loader (guarding the unwrapExports
export-shape bug class, postmortem 0001). ACP snapshot replay stays green
against the existing committed goldens (pure boot restructuring). RFC moved
proposed->implemented with the amendment recorded; package/example/architecture
docs and the module graph updated.

packages/ui/stdio-agent/README.md

@@ -0,0 +1,60 @@
+# @deepseek-ai/dsh-stdio-agent
+
+The **terminal stdio chat app**: a Cordis app plugin that composes the providerless agent spine ([`@deepseek-ai/dsh-agent-core`](../../core/agent-core/README.md)) with the front-door cluster a terminal chat needs, and a `bin` that boots a leaf `cordis.yml`.
+
+It is the readline counterpart to [`@deepseek-ai/dsh-acp-agent`](../acp-agent/README.md): both consume the same spine, but each bakes in the OPPOSITE front-door cluster.
+
+## What it bakes in
+
+A terminal chat always wants the same cluster, so the package owns it rather than trusting each leaf to re-wire it:
+
+| Plugin | Why it is here |
+|---|---|
+| `@cordisjs/plugin-logger-console` | the console logger — stdout is just the terminal here, so logging to it is correct (the ACP app must NOT have this) |
+| `@deepseek-ai/dsh-agent-core` | the spine, pre-creating a `main` agent from this app's `model`/`systemPrompt` |
+| `@deepseek-ai/dsh-session-persistence-jsonl` | durable JSONL session log under `persistenceRoot` |
+| `@deepseek-ai/dsh-ui-stdio` | the readline UI, bound to the `main` agent |
+
+`@cordisjs/plugin-hmr` (the dev/demo edit-reload loop) is deliberately a **leaf** entry, NOT baked in here: it is a Loader-only, subprocess-only dev plugin — its constructor throws without `node --expose-internals` + a live `loader`, and the in-process test tier cannot even import it (so a package whose `apply` statically pulled it in could never carry the per-file coverage gate). Unlike the console logger, a stray `hmr` is not a stdout-purity footgun, so leaving it at the leaf costs no safety. The `demo:echo` / `demo:coding` leaves load it and pass `--expose-internals`.
+
+The leaf `cordis.yml` supplies only the **swappable backends** — an LLM adapter (`llm-deepseek` for the real model, or the mock `mock-llm` for a demo) and a bash executor (`bash-local`) — `hmr`, plus this app's [`Config`](#config). The whole plugin tree a run loads is therefore: this app's cluster, the spine inside `agent-core`, `hmr`, and the two leaf backends.
+
+## Config
+
+| Key | Default | Routed to |
+|---|---|---|
+| `model` | (required) | the pre-created `main` agent's model |
+| `systemPrompt` | (required) | the `main` agent's system prompt |
+| `persistenceRoot` | `./.sessions` | the JSONL backend's root directory |
+| `welcome` | `ready.` | the stdin-chat banner |
+| `resumeSessionId` | — | resume a persisted session id instead of starting fresh (sourced from an env var in the leaf) |
+
+## The bin
+
+`dsh-stdio-agent [path-to-cordis.yml]` (default `./cordis.yml`) loads a gitignored `.env` from the cwd (`DEEPSEEK_API_KEY` / `DEEPSEEK_BASE_URL`), then drives the cordis Loader against the config — the boot glue the `examples/*/start.ts` files once each duplicated. The `demo:echo` / `demo:coding` scripts invoke it.
+
+## Example leaf `cordis.yml`
+
+```yaml
+# A real coding agent: hmr + the DeepSeek adapter + local bash, then this app.
+- id: hmr
+  name: '@cordisjs/plugin-hmr'
+  config:
+    root: ['.']
+- id: llm-deepseek
+  name: '@deepseek-ai/dsh-llm-deepseek'
+  config:
+    apiKey: !!js process.env.DEEPSEEK_API_KEY
+    models: [deepseek-v4-flash]
+- id: bash
+  name: '@deepseek-ai/dsh-bash-local'
+  config:
+    timeoutMs: 60000
+- id: stdio-agent
+  name: '@deepseek-ai/dsh-stdio-agent'
+  config:
+    model: deepseek-v4-flash
+    systemPrompt: 'You are a CLI coding assistant. Your only tools are bash…'
+```
+
+Swap `llm-deepseek` for a `mock-llm` leaf plugin and you have the echo demo — "swap the backend, keep the app".