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/echo-agent/tests/echo.e2e.ts

@@ -7,22 +7,29 @@ import { afterEach, describe, expect, it } from 'vitest'
 
 /**
  * Keyless Loader-path smoke for examples/echo-agent: boot the REAL example
- * through its `cordis.yml` (the cordis Loader, `unwrapExports`, the whole
- * plugin tree), pipe a script of stdin lines, and assert the rendered stdout.
+ * through the `@deepseek-ai/dsh-stdio-agent` bin against this example's
+ * `cordis.yml` (the cordis Loader, `unwrapExports`, the whole plugin tree),
+ * pipe a script of stdin lines, and assert the rendered stdout.
  *
  * This is the guard the per-file unit suite structurally cannot be: it drives
- * the extracted `@deepseek-ai/dsh-ui-stdio` plugin AND the example-local
- * `mock-llm.ts` / `echo-tool.ts` through their REAL load path, so a broken
- * plugin export shape (a stray `export default` that `unwrapExports` would
- * collapse, dropping `inject`) fails here even though hand-mounted unit tests
- * stay green (see docs/postmortem/0001). It needs no API key — the `mock-echo`
- * adapter never touches the network — so it runs in the default e2e gate.
+ * the `@deepseek-ai/dsh-stdio-agent` app plugin, the `@deepseek-ai/dsh-agent-core`
+ * bundle it loads, the extracted `@deepseek-ai/dsh-ui-stdio` plugin, AND the
+ * example-local `mock-llm.ts` / `echo-tool.ts` through their REAL load path, so
+ * a broken plugin export shape (a stray `export default` that `unwrapExports`
+ * would collapse, dropping `inject`/`Config`) fails here even though hand-mounted
+ * unit tests stay green (see docs/postmortem/0001). It needs no API key — the
+ * `mock-echo` adapter never touches the network — so it runs in the default e2e
+ * gate.
  *
  * Both branches of mock-llm.ts are exercised: an `echo …` line (the tool
  * round-trip → `ECHO: …`) and a plain line (the direct canned reply).
  */
 
-const startScript = fileURLToPath(new URL('../start.ts', import.meta.url))
+// The dsh-stdio-agent bin (the demo:echo entry) and this example's cordis.yml.
+// The bin resolves its config-path arg from CWD; the test spawns from a temp
+// cwd, so we pass the example config's ABSOLUTE path.
+const binScript = fileURLToPath(new URL('../../../packages/ui/stdio-agent/src/bin.ts', import.meta.url))
+const configPath = fileURLToPath(new URL('../cordis.yml', import.meta.url))
 const tsxLoader = fileURLToPath(import.meta.resolve('tsx'))
 // Dev/test run UNBUILT: `@deepseek-ai/dsh-*` imports resolve through the root
 // tsconfig `paths` map, which tsx finds by searching UP from cwd. We spawn from
@@ -53,8 +60,8 @@ async function runEcho(lines: string[]): Promise<{ stdout: string; code: number
       process.execPath,
       // --expose-internals: the example's cordis.yml loads the HMR plugin, which
       // requires it (mirrors the `demo:echo` script). The whole point is to boot
-      // the example EXACTLY as it really runs, through the Loader.
-      ['--expose-internals', '--import', tsxLoader, startScript],
+      // the example EXACTLY as it really runs, through the bin + Loader.
+      ['--expose-internals', '--import', tsxLoader, binScript, configPath],
       { cwd, env: { ...process.env, TSX_TSCONFIG_PATH: repoTsconfig }, stdio: ['pipe', 'pipe', 'pipe'] },
     )
     child = proc