refactor(examples): extract reusable logic into tested packages

Logic that lived under examples/ was outside the per-file 100% coverage
gate (examples/ are not workspaces) and, in the stdio-UI case, duplicated
across two examples. Move it into packages/ so it is gated and de-duped.

- packages/ui-stdio (new): unify the two diverged stdio-chat.ts copies into
  one @deepseek-ai/dsh-ui-stdio plugin (welcome/agent Config). A test-only
  I/O seam (createStdioChat(ctx, config, runtime)) keeps process streams out
  of the serializable config and makes every render/EOF/disposal branch
  unit-testable. Per-file 100%. echo/coding cordis.yml now load the package;
  both src/stdio-chat.ts deleted.
- packages/llm-replay (new): move examples/acp-agent/src/llm-replay.ts (+ its
  spec) here so its derive/parse/replay branches fall under the coverage gate.
  cordis.snapshot.yml + README rewired to the package name; added apply/env
  /assertNever/abort tests to reach per-file 100%.
- examples/{echo,coding}-agent: keyless Loader-path e2e smokes that boot the
  real cordis.yml (no key) — the guard a hand-mounted unit test cannot be for
  the unwrapExports/export-shape class (postmortem 0001). examples/AGENTS.md
  codifies the keyless+with-key smoke convention (keyless-by-nature exception
  for echo-agent).
- AGENTS.md: a scoped, removal-triggered pre-release stance (foundation over
  blast radius). packages/README.md: new rows + a FIXME to later regroup ALL
  packages into a hierarchy. Wiring: tsconfig paths/refs, publint, knip,
  module-graph.

Verified: typecheck, lint, test:coverage (887 tests, 100%), build, hygiene,
doc-sync, test:snapshot (10), test:e2e (6 keyless pass, with-key self-skip).

examples/coding-agent/tests/keyless-smoke.e2e.ts

@@ -0,0 +1,92 @@
+import { spawn, type ChildProcessWithoutNullStreams } from 'node:child_process'
+import { mkdtemp, rm } from 'node:fs/promises'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import { fileURLToPath } from 'node:url'
+import { afterEach, describe, expect, it } from 'vitest'
+
+/**
+ * Keyless Loader-path smoke for examples/coding-agent: boot the REAL example
+ * through its `cordis.yml` (the cordis Loader, `unwrapExports`, the full plugin
+ * tree incl. the extracted `@deepseek-ai/dsh-ui-stdio`), then close stdin with
+ * no prompt and assert the ready banner + a clean exit.
+ *
+ * No prompt is ever sent, so the model is NEVER called — this is why it runs
+ * without a real key. coding-agent's `cordis.yml` loads `llm-deepseek`, whose
+ * `apply()` only requires a key to be PRESENT (it does not validate it and only
+ * uses it when a stream actually starts), so a dummy key lets the tree boot
+ * while the absence of any prompt guarantees no network call. The value is the
+ * real-Loader-path guard for the shared UI plugin's export shape (a broken
+ * `export default` that drops `inject` would crash here — see postmortem 0001),
+ * complementing coding-agent's with-key e2e suites which prove the real product.
+ */
+
+const startScript = fileURLToPath(new URL('../start.ts', import.meta.url))
+const tsxLoader = fileURLToPath(import.meta.resolve('tsx'))
+// Dev/test run UNBUILT: resolve `@deepseek-ai/dsh-*` through the root tsconfig
+// `paths` map; tsx searches UP from cwd, and we spawn from a temp dir outside
+// the repo, so point it at the repo tsconfig (root is four levels up).
+const repoTsconfig = fileURLToPath(new URL('../../../tsconfig.json', import.meta.url))
+
+let child: ChildProcessWithoutNullStreams | undefined
+let workdir: string | undefined
+
+afterEach(async () => {
+  if (child !== undefined && child.exitCode === null) child.kill('SIGKILL')
+  child = undefined
+  if (workdir !== undefined) await rm(workdir, { recursive: true, force: true })
+  workdir = undefined
+})
+
+async function bootAndEof(): Promise<{ stdout: string; code: number }> {
+  workdir = await mkdtemp(join(tmpdir(), 'coding-smoke-'))
+  const cwd = workdir
+  return new Promise((resolve, reject) => {
+    const proc = spawn(
+      process.execPath,
+      // --expose-internals: cordis.yml loads the HMR plugin (mirrors demo:coding).
+      ['--expose-internals', '--import', tsxLoader, startScript],
+      {
+        cwd,
+        env: {
+          ...process.env,
+          TSX_TSCONFIG_PATH: repoTsconfig,
+          // A dummy key so llm-deepseek's apply() (key-PRESENT check only) boots.
+          // No prompt is sent, so the adapter never streams — no network call.
+          DEEPSEEK_API_KEY: 'keyless-smoke-no-call',
+        },
+        stdio: ['pipe', 'pipe', 'pipe'],
+      },
+    )
+    child = proc
+    let stdout = ''
+    let stderr = ''
+    proc.stdout.setEncoding('utf8')
+    proc.stdout.on('data', (chunk: string) => { stdout += chunk })
+    proc.stderr.setEncoding('utf8')
+    proc.stderr.on('data', (chunk: string) => { stderr += chunk })
+
+    const timer = setTimeout(() => {
+      proc.kill('SIGKILL')
+      reject(new Error(`coding-agent did not exit within 10s. stdout:\n${stdout}\nstderr:\n${stderr}`))
+    }, 10_000)
+
+    proc.on('exit', (code) => {
+      clearTimeout(timer)
+      if (code === 0) resolve({ stdout, code })
+      else reject(new Error(`coding-agent exited ${code}. stderr:\n${stderr}`))
+    })
+    proc.on('error', (err) => { clearTimeout(timer); reject(err) })
+
+    // No prompt — just EOF, so the stdio UI exits without ever running a turn.
+    proc.stdin.end()
+  })
+}
+
+describe('coding-agent keyless smoke (real cordis.yml via the Loader)', () => {
+  it('boots the full plugin tree, prints its banner, and exits cleanly on EOF', async () => {
+    const { stdout, code } = await bootAndEof()
+    expect(code).toBe(0)
+    expect(stdout).toContain('coding-agent ready.')
+  }, 15_000)
+})