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.

docs/core-data-structures/core.md

@@ -140,6 +140,20 @@ interface GenerateOptions {
    */
   stop?: string[]
   signal?: AbortSignal
+  /**
+   * The id of the session this request belongs to — stamped by the agent loop
+   * from `agent.session.id`. Adapters ignore it; it lets an `llm/stream` listener
+   * route a call by WHICH session issued it (the replay adapter keys its per-call
+   * cursor by session, so a parent and its in-process subagent — each with its
+   * own session on one context — replay from their own recorded scripts).
+   *
+   * Typed as `Branded<'SessionId'>` rather than importing `SessionId` from
+   * `dsh-session`: that package imports `Message` from here, so importing its
+   * `SessionId` back would cycle. `SessionId` IS `Branded<'SessionId'>`, so a
+   * real session id assigns with no cast. (A future ids package could own the
+   * brand and dissolve this note.)
+   */
+  sessionId?: Branded<'SessionId'>
 }
 ```