feat(acp-example): snapshot harness, normalizers, wiring, and handshake scenario

Adds the snapshot-test harness and the keyless replay pipeline end-to-end.

- snapshot-harness.ts: boots the real acp-agent subprocess via the cordis
  Loader (preserving TSX_TSCONFIG_PATH so unbuilt dsh-* imports resolve from a
  temp cwd), tees raw stdout into an SDK ClientSideConnection, interprets a
  per-scenario input.json DSL (initialize / newSession capturing the random
  sessionId / prompt / cancel), closes stdin to trigger graceful shutdown, and
  harvests the persisted session.jsonl. Failure-safe: a finally block SIGKILLs
  a live child, awaits its exit, and removes both temp dirs even on a thrown
  step or harvest. Raw bytes are buffered and decoded once (no multibyte split).
- snapshot-normalize.ts (+ spec): two pure normalizers (stdout frames + session
  JSONL) scrub cwd, session ids / UUIDs, and JSON-RPC ids, and zero time /
  createdAt — but keep `seq` (deterministic by contract). normalizeStdout throws
  on a non-JSON line (the stdout-purity check).
- start.ts: selects cordis.snapshot.yml (replay, providerless) or
  cordis.snapshot-record.yml (record, real adapter) from DSH_SNAPSHOT, skips
  .env in replay, and disposes the ctx on stdin end so persistence flushes
  before exit (harvest-after-flush, not on the prompt response).
- acp.snapshot.ts: asserts the normalized stdout golden (and, for model
  scenarios, the re-persisted JSONL golden) via toMatchFileSnapshot; record mode
  writes the harvested log back to the scenario fixture; an orphan-fixture guard
  fails on an unregistered scenario dir.
- handshake scenario: initialize + session/new (no model call; a header-only
  session.jsonl, since session/new persists no events).
- vitest.snapshot.config.ts, test:snapshot / test:snapshot:record scripts, a
  pre-push snapshot job, and the knip entry.

Incorporates Codex review: record-fixture writeback, failure-safe teardown,
seq-not-scrubbed, harvest-after-flush. Per docs/rfc/implemented/2026-06-19.

examples/acp-agent/cordis.snapshot-record.yml

@@ -0,0 +1,43 @@
+# Snapshot-test RECORD config: a real run whose persisted session JSONL is
+# harvested into a scenario fixture. Identical to cordis.yml (real llm-deepseek
+# adapter + JSONL persistence) — recording must exercise the REAL model so the
+# recorded log is a genuine product of the system. Needs DEEPSEEK_API_KEY.
+#
+# It is a separate file (rather than reusing cordis.yml) only so the snapshot
+# harness selects it explicitly via $DSH_SNAPSHOT=record and so its persistence
+# root can be pointed at the harness's harvest directory by the same env the
+# replay path uses. The graceful-shutdown path in start.ts flushes persistence
+# before exit so the harvested log is complete.
+
+- id: timer
+  name: '@cordisjs/plugin-timer'
+
+# Shared provider/tool core, INCLUDING the real llm-deepseek adapter.
+- id: base
+  name: '@cordisjs/plugin-include'
+  config:
+    path: '../base.yml'
+
+- id: agent-loop
+  name: '@deepseek-ai/dsh-agent-loop'
+  config:
+    agents: []
+
+- id: session-persistence
+  name: '@deepseek-ai/dsh-session-persistence-jsonl'
+  config:
+    root: !!js process.env.DSH_SNAPSHOT_SESSIONS_ROOT
+
+- id: acp
+  name: '@deepseek-ai/dsh-acp'
+  config:
+    model: deepseek-v4-flash
+    systemPrompt: |
+      You are a coding assistant driven over the Agent Client Protocol.
+
+      Your only tools are bash (plus bash_output/bash_kill for background
+      tasks). Do ALL file operations through bash: read with cat/sed/head,
+      search with grep, write with heredocs (cat <<'EOF' > file), edit with
+      sed or a rewrite. Each bash call runs in a fresh shell — pass workdir
+      instead of cd. Check the [exit code: N] marker; verify your work. Keep
+      answers brief and factual.