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).

docs/module-graph.md

@@ -14,6 +14,8 @@ graph TD
   system-prompt --> llm
   agent --> llm
   agent --> session
+  llm-replay --> llm
+  llm-replay --> session
   session-persistence --> session
   invariants --> agent
   invariants --> llm
@@ -25,6 +27,9 @@ graph TD
   tools --> agent
   tools --> llm
   tools --> system-prompt
+  ui-stdio --> agent
+  ui-stdio --> llm
+  ui-stdio --> session
   acp --> agent
   acp --> llm
   acp --> session
@@ -52,11 +57,13 @@ graph TD
 | `session` | `llm` |
 | `system-prompt` | `llm` |
 | `agent` | `llm`, `session` |
+| `llm-replay` | `llm`, `session` |
 | `session-persistence` | `session` |
 | `invariants` | `agent`, `llm`, `session` |
 | `session-persistence-jsonl` | `session`, `session-persistence` |
 | `session-persistence-sqlite` | `session`, `session-persistence` |
 | `tools` | `agent`, `llm`, `system-prompt` |
+| `ui-stdio` | `agent`, `llm`, `session` |
 | `acp` | `agent`, `llm`, `session`, `session-persistence`, `tools` |
 | `agent-loop` | `agent`, `llm`, `session`, `session-persistence`, `system-prompt`, `tools` |
 | `tool-bash` | `agent`, `bash`, `llm`, `tools` |