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.yml

@@ -0,0 +1,69 @@
+# Snapshot-test REPLAY config: the acp-agent plugin tree with the model replaced
+# by llm-replay (serves a recorded session JSONL — no API key, no network).
+#
+# This does NOT include ../base.yml: base.yml always loads
+# @deepseek-ai/dsh-llm-deepseek, whose apply() throws without DEEPSEEK_API_KEY,
+# so a keyless replay run would die at boot. We inline the providerless core
+# instead and install llm-replay where the adapter would be.
+#
+# stdout is reserved for the ACP JSON-RPC protocol — no stdout logger (see
+# cordis.yml). The replay fixture path comes from $DSH_SNAPSHOT_FILE (and an
+# optional $DSH_SNAPSHOT_OVERRIDE sidecar), set by the snapshot harness.
+
+- id: timer
+  name: '@cordisjs/plugin-timer'
+
+# Providerless core (everything base.yml has EXCEPT llm-deepseek).
+- id: llm
+  name: '@deepseek-ai/dsh-llm'
+
+- id: sessions
+  name: '@deepseek-ai/dsh-session'
+
+- id: system-prompt
+  name: '@deepseek-ai/dsh-system-prompt'
+
+- id: tools
+  name: '@deepseek-ai/dsh-tools'
+
+- id: agents
+  name: '@deepseek-ai/dsh-agent'
+
+- id: invariants
+  name: '@deepseek-ai/dsh-invariants'
+
+- id: bash
+  name: '@deepseek-ai/dsh-bash-local'
+  config:
+    timeoutMs: 60000
+
+- id: tool-bash
+  name: '@deepseek-ai/dsh-tool-bash'
+
+# The replay adapter: short-circuits llm/stream with the recorded log's chunks.
+- id: llm-replay
+  name: './src/llm-replay.ts'
+
+- 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.