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).

packages/ui-stdio/README.md

@@ -0,0 +1,42 @@
+# @deepseek-ai/dsh-ui-stdio
+
+A minimal stdio (readline) UI, as a plugin. It reads lines from stdin and feeds them to an agent (`send` when idle, `steer` while a turn is running), and renders that agent's streamed output and tool activity to stdout. A UI is "just a plugin" here — it only consumes the `agent/*` event taxonomy plus the `agents` service (`inject: ['agents']`), so the same plugin drives any example or product surface.
+
+This package consolidates what were two near-identical copies under `examples/echo-agent` and `examples/coding-agent`. The coding copy was a superset; this package IS that superset — dimmed chain-of-thought rendering plus robust piped-stdin EOF handling — with the per-consumer differences moved into `Config`.
+
+## Config
+
+| Key | Type | Default | Notes |
+|---|---|---|---|
+| `welcome` | string | `'ready.'` | Banner printed once on start, before the first `> ` prompt. |
+| `agent` | string | `'main'` | Id of the agent to drive and render. |
+
+```yaml
+- id: ui-stdio
+  name: '@deepseek-ai/dsh-ui-stdio'
+  config:
+    welcome: 'coding-agent ready. Give it a coding task.'
+```
+
+## Rendering
+
+- `agent/stream-chunk` — `text-delta` is written verbatim; `reasoning-delta` is wrapped in the dim SGR (`\x1B[2m … \x1B[0m`) so the chain-of-thought is visually subordinate to the answer. Reasoning rendering is inert when no `reasoning-delta` chunks arrive (e.g. a mock model), so it is always on.
+- `agent/turn-start` / `agent/turn-end` — a `[<agent> turn N]` header and a trailing `> ` prompt.
+- `session/event` — `tool/call` renders `[tool call] name(args)`; `tool/result` renders the joined text blocks as `[tool result] …`.
+
+## The I/O seam
+
+The production entry point `apply(ctx, config)` binds the real `process` streams. The testable core is `createStdioChat(ctx, config, runtime)`, where `runtime: StdioRuntime` supplies `input` / `output` / `exit`. This seam is deliberately **not** part of the serializable `Config` (streams and functions do not belong in YAML config); it exists so the render, EOF, and disposal branches can be exercised with fakes instead of hijacking globals.
+
+## Piped-stdin exit
+
+On stdin EOF the plugin exits the process, but carefully:
+
+- **No work submitted** (empty stdin, blank-only lines): exit immediately — no turn will ever start, so there is nothing to wait for. Gating on an observed `running` here would hang forever.
+- **Work submitted**: exit the next time the agent settles to `idle` *after* having been observed `running`. `agent.send()` does not synchronously flip status to `running`, so requiring an observed `running` first (`sawRunning`) avoids exiting in the gap before the turn starts and dropping work; and the loop batches several queued messages into one turn, so the exit keys off the idle transition rather than counting sends.
+
+Disposal (HMR or fiber teardown) closes the readline interface, which also fires `close` — a `disposed` guard ensures teardown never calls `process.exit`.
+
+## Plugin export shape
+
+Named `name` / `inject` / `Config` / `apply`, with **no default export**: the cordis Loader's `unwrapExports` does `exports.default ?? exports`, so a stray default would collapse the module to the bare function and drop the `inject` namespace (see [docs/postmortem/0001](../../docs/postmortem/0001-acp-default-export-drops-inject.md)). The keyless Loader-path e2e smokes in `examples/{echo,coding}-agent` guard this end-to-end.