Persist the seed boundary so fork-child replay routes correctly

A fork subagent seeds its child session with a prefix of the parent's log, and
that seed becomes the child's persisted log — so a fork child's .jsonl begins
with the PARENT's events, including the parent's assistant/chunk events. The
snapshot replay harness derived a child's script from its whole log, which would
replay the parent's recorded responses as the child's model calls. Spawn-only
scenarios never hit it, but a fork snapshot would mis-route silently.

Record the seed boundary and skip the inherited prefix at replay:

- SessionHeader gains an optional `seedLength` (how many leading events were
  inherited via a seed), threaded through CreateSessionOptions/CreateAgentOptions
  meta and stamped by the fork backend (= seeded-prefix length; absent for spawn).
  It is EXPLICIT, never inferred from seed.length: a resume seeds the whole stored
  log, so the resume path passes the persisted boundary back.
- Both persistence backends round-trip it: JSONL header line, SQLite seed_length
  column. The SQLite table change bumps SCHEMA_VERSION 2->3; per the pre-release
  stance the backend rejects an older user_version on open with NO migration.
- llm-replay's parseSessionHeader reads seedLength and loadSessionScripts derives
  a child script from events AFTER the boundary. seedLength is 0 for spawn, so
  spawn replay is byte-for-byte unchanged.

Closes the routing-correctness gap the per-session snapshot replay RFC under-
stated; a recorded fork scenario remains a future addition but now derives
correctly. RFC: docs/rfc/implemented/testing/2026-06-22-fork-child-replay-seed-boundary.md.

Regression coverage: a fork child fixture whose seeded prefix carries a parent
chunk (derived script must exclude it, proven red without the slice); a seedLength
persistence round-trip through the shared coordinator contract (both backends);
the fork backend stamping it; resume preserving it from the persisted header.

packages/session-persistence/session-persistence-sqlite/src/index.ts

@@ -224,19 +224,21 @@ export class SessionPersistenceSqlite extends SessionPersistence implements Pers
    */
   private writeRow(meta: SessionHeader): void {
     this.db.prepare(`
-      INSERT INTO sessions (id, version, created_at, cwd, parent_session)
-      VALUES (?, ?, ?, ?, ?)
+      INSERT INTO sessions (id, version, created_at, cwd, parent_session, seed_length)
+      VALUES (?, ?, ?, ?, ?, ?)
       ON CONFLICT(id) DO UPDATE SET
         version = excluded.version,
         created_at = excluded.created_at,
         cwd = excluded.cwd,
-        parent_session = excluded.parent_session
+        parent_session = excluded.parent_session,
+        seed_length = excluded.seed_length
     `).run(
       meta.id,
       meta.version,
       meta.createdAt,
       meta.cwd ?? null,
       meta.parentSession ?? null,
+      meta.seedLength ?? null,
     )
   }
 }