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.

examples/acp-agent/tests/acp.e2e.ts

@@ -26,7 +26,11 @@ import {
  * WITHOUT a key, since it only needs the server to boot and answer initialize.
  */
 
-const startScript = fileURLToPath(new URL('../start.ts', import.meta.url))
+// The dsh-acp-agent bin (the demo:acp entry) and this example's cordis.yml. The
+// bin resolves its config-path arg from CWD; the subprocess runs from a temp
+// workdir, so pass the example config's ABSOLUTE path.
+const binScript = fileURLToPath(new URL('../../../packages/ui/acp-agent/src/bin.ts', import.meta.url))
+const configPath = fileURLToPath(new URL('../cordis.yml', import.meta.url))
 // Resolve tsx's loader to an ABSOLUTE path: the subprocess runs with cwd set to
 // a temp workdir (this test launches there and uses it as the session cwd; the
 // bridge no longer requires cwd === the launch dir, but a temp dir keeps the
@@ -55,7 +59,7 @@ interface Spawned {
 function spawnAcpAgent(cwd: string, env: NodeJS.ProcessEnv = process.env): Spawned {
   const child = spawn(
     process.execPath,
-    ['--import', tsxLoader, startScript],
+    ['--import', tsxLoader, binScript, configPath],
     { cwd, env: { ...env, TSX_TSCONFIG_PATH: repoTsconfig }, stdio: ['pipe', 'pipe', 'pipe'] },
   )
   const stderr: string[] = []
@@ -101,7 +105,7 @@ describe('acp-agent over real stdio (no key required)', () => {
     // A dummy key lets the deepseek adapter APPLY (it only checks the key is
     // present at boot, not valid — the key is used only on a real model call,
     // which this purity test never triggers). So this runs WITHOUT real creds.
-    const child = spawn(process.execPath, ['--import', tsxLoader, startScript], {
+    const child = spawn(process.execPath, ['--import', tsxLoader, binScript, configPath], {
       cwd: workdir,
       env: { ...process.env, DEEPSEEK_API_KEY: process.env.DEEPSEEK_API_KEY ?? 'sk-dummy-for-boot', TSX_TSCONFIG_PATH: repoTsconfig },
       stdio: ['pipe', 'pipe', 'pipe'],

examples/acp-agent/tests/snapshot-harness.ts

@@ -31,7 +31,12 @@ import {
   type SessionNotification,
 } from '@agentclientprotocol/sdk'
 
-const startScript = fileURLToPath(new URL('../start.ts', import.meta.url))
+// The dsh-acp-agent bin (the demo:acp entry) and this example's cordis.yml.
+// The bin resolves its config-path arg from CWD and, under DSH_SNAPSHOT=replay,
+// swaps it for the sibling cordis.snapshot.yml. The child's cwd is a temp dir
+// OUTSIDE the repo, so pass the example config's ABSOLUTE path.
+const binScript = fileURLToPath(new URL('../../../packages/ui/acp-agent/src/bin.ts', import.meta.url))
+const configPath = fileURLToPath(new URL('../cordis.yml', import.meta.url))
 const tsxLoader = fileURLToPath(import.meta.resolve('tsx'))
 // The repo-root tsconfig: dev/test run UNBUILT and the `@deepseek-ai/dsh-*`
 // imports resolve through its `paths` map. The child's cwd is a temp dir
@@ -130,7 +135,7 @@ export async function runScenario(input: InputScript, opts: RunOptions): Promise
 
     child = spawn(
       process.execPath,
-      ['--import', tsxLoader, startScript],
+      ['--import', tsxLoader, binScript, configPath],
       { cwd, env, stdio: ['pipe', 'pipe', 'pipe'] },
     )