Add per-session snapshot replay for nested agents (PR2.5)

The snapshot tier was built single-session: dsh-llm-replay served calls from
one global positional cursor, and the harness harvested one session log. A
subagent runs as a second agent with its own session, so a parent→child
scenario could neither replay deterministically nor harvest the child's log.
This resolves the TODO(subagent-snapshots) deferral from the subagent RFC.

- Stamp the calling session id onto the model request: GenerateOptions.sessionId
  (typed Branded<'SessionId'> to avoid the dsh-llm↔dsh-session cycle), set by the
  agent loop from agent.session.id. Adapters ignore it; an llm/stream listener
  routes by it.
- Key replay per session: dsh-llm-replay loads the parent log plus one per child
  (childFiles / $DSH_SNAPSHOT_CHILD_FILES), derives a script per recorded session,
  and binds each live (freshly-random) session to a recorded script by first-call
  order — parent first (earliest createdAt, first to stream). Keys by WHO calls,
  so it survives a future concurrent/backgrounded subagent; a global cursor would
  not. An unrecorded extra session fails loud.
- Harvest every log: the harness collects all .jsonl across cwd buckets, ordered
  primary-first (top-level, then children by createdAt), and RunResult exposes the
  plural sessionLogs. The spec writes each back on record (session.jsonl +
  session.<n>.jsonl) and diffs each against its fixture on replay.
- Wire the subagent seam + spawn + fork + tool into the acp-agent example (both
  cordis configs) and add two nested scenarios recorded against the real API:
  subagent-spawn (parent + 1 child) and subagent-multi (parent + 2 children, 3
  sessions). Both replay keyless in the default gate.

A new RFC documents the design (docs/rfc/implemented/testing/). Single-session
replay is unchanged (a call with no sessionId is one anonymous primary session).

TODO follow-up: a dedicated branded-ids package could own the SessionId brand and
dissolve the cross-package cycle note; out of scope for this testing PR.

examples/acp-agent/cordis.snapshot.yml

@@ -31,9 +31,33 @@
     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.
+      Your tools are bash (plus bash_output/bash_kill for background tasks)
+      and subagent. 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.
+
+      Use the subagent tool to delegate a focused, self-contained subtask to
+      a fresh child agent (it works in its own context and returns only its
+      final result) — give it a complete, standalone instruction.
+
+# The subagent seam + both in-process backends + the model-facing `subagent`
+# tool — identical to cordis.yml's wiring (only the LLM backend differs above).
+- id: subagent
+  name: '@deepseek-ai/dsh-subagent'
+
+- id: subagent-spawn
+  name: '@deepseek-ai/dsh-subagent-spawn'
+  config:
+    providerName: spawn
+
+- id: subagent-fork
+  name: '@deepseek-ai/dsh-subagent-fork'
+  config:
+    providerName: fork
+
+- id: tool-subagent
+  name: '@deepseek-ai/dsh-tool-subagent'
+  config:
+    provider: spawn