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/acp-agent/README.md

@@ -0,0 +1,39 @@
+# @deepseek-ai/dsh-acp-agent
+
+The **ACP server 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 an [Agent Client Protocol](../acp/README.md) server needs, and a `bin` that boots a leaf `cordis.yml` speaking ACP JSON-RPC on stdio.
+
+It is the structured counterpart to [`@deepseek-ai/dsh-stdio-agent`](../stdio-agent/README.md): both consume the same spine, but this one bakes in the OPPOSITE front-door cluster.
+
+## What it bakes in — and what it deliberately omits
+
+stdout is the ACP JSON-RPC channel, so the cluster is defined as much by what it LEAVES OUT as what it includes:
+
+| Plugin | Why |
+|---|---|
+| `@deepseek-ai/dsh-agent-core` | the spine, pre-creating **no** agents (ACP `session/new` creates them on demand) |
+| `@deepseek-ai/dsh-session-persistence-jsonl` | durable JSONL session log (the bridge advertises `loadSession`) |
+| `@deepseek-ai/dsh-acp` | the bridge that owns stdout for JSON-RPC |
+| ~~console logger~~ | **omitted** — it writes to stdout and would corrupt the protocol frames ([the stdout-purity footgun](../acp/README.md)) |
+| ~~`hmr`~~ | **omitted** — the editor owns the subprocess |
+
+Because there is no logger entry in the package, the footgun is **structurally unreachable from the leaf**: a leaf author cannot wire a stdout logger into the ACP config, because the leaf only picks backends, not the front door.
+
+## Config
+
+| Key | Default | Routed to |
+|---|---|---|
+| `model` | (required) | the per-session agent template the bridge creates agents from |
+| `systemPrompt` | (required) | the per-session agent's system prompt |
+| `persistenceRoot` | `./.sessions` | the JSONL backend's root directory |
+
+The leaf supplies the swappable backends: an LLM adapter (`llm-deepseek` for the real model, `llm-replay` for keyless snapshot replay) and a bash executor (`bash-local`).
+
+## The bin
+
+`dsh-acp-agent [path-to-cordis.yml]` (default `./cordis.yml`):
+
+- loads a gitignored `.env` from the cwd — **skipped** in snapshot REPLAY so a stray key can never trigger a live call;
+- honors `DSH_SNAPSHOT=replay` by booting the sibling `cordis.snapshot.yml` (the keyless replay tree, `llm-replay` in place of `llm-deepseek`);
+- in a snapshot run, disposes the context on stdin EOF so the session log is fully flushed before exit.
+
+All diagnostics go to **stderr** — stdout is the protocol.