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/rfc/proposed/feature/2026-06-21-subagent-capability-seam.md

@@ -70,4 +70,4 @@ The `dsh-tool-subagent` consumer awaits `run.result` and returns the child's fin
 - **Blocking the parent turn.** Synchronous collect holds the parent's `runStep` open for the child's full duration. This is acceptable for the first cut; **background / poll / spill semantics are deferred to a future redesign that unifies long-running-tool handling across subagents AND bash** (a sub-agent and a long `bash` background task pose the same "the model started something slow, how does it collect later" problem, and should share one mechanism rather than each inventing its own).
 - **Live progress.** This cut surfaces only lifecycle + final result; a per-chunk child→parent update stream is deferred with the background redesign.
 - **ACP client surface.** Proxying `fs`/`terminal` from the ACP child back to the parent (a shared-workspace mode) is future work; the first cut advertises neither, so the child self-serves in its own process.
-- **Snapshot coverage of nested agents.** The snapshot tier (`pnpm run test:snapshot`) replays a recorded session through `dsh-llm-replay`, whose dispatch is a single GLOBAL positional cursor (the Nth `llm/stream` call serves the Nth recorded entry) and whose harness harvests a single session log file. A subagent runs as a *second* agent with its own session log, so a parent→child scenario needs per-session-keyed replay (or a call-ordered merge of both logs, sound because subagent execution is strictly nested/non-concurrent — the parent blocks on the child) plus harvest-all-logs and plural-session-id plumbing in the harness. This is self-contained infrastructure orthogonal to the backends, so it lands as a **dedicated stacked follow-up** rather than in the in-process-backends PR. Until it lands, in-process subagents are covered by real-loop unit tests (a parent driving a fork AND a spawn child) and a with-key e2e (a parent delegating to a child that writes a file), not by the snapshot transcript tier. Tracked by `TODO(subagent-snapshots)`.
+- **Snapshot coverage of nested agents.** The snapshot tier (`pnpm run test:snapshot`) replays a recorded session through `dsh-llm-replay`. It was built single-session: a single GLOBAL positional cursor (the Nth `llm/stream` call serves the Nth recorded entry) and a harness that harvested a single session log file. A subagent runs as a *second* agent with its own session log, so a parent→child scenario needed per-session-keyed replay plus harvest-all-logs and plural-session-id plumbing — self-contained infrastructure orthogonal to the backends, scheduled as a dedicated stacked follow-up rather than folded into the in-process-backends PR. That follow-up has **landed**: see [Per-session snapshot replay for nested agents](../../implemented/testing/2026-06-22-subagent-snapshot-replay.md). Replay now keys each call by its calling session (`GenerateOptions.sessionId`) and binds live sessions to recorded scripts by first-call order; the harness harvests every log; and two nested scenarios (`subagent-spawn`, `subagent-multi`) replay keyless in the default gate. In-process subagents remain covered by real-loop unit tests and a with-key e2e in addition to the snapshot tier.