refactor(sidechat): 删除零引用的 sidechatSeed

src/sidechat-core.ts 的 sidechatSeed()（buildSidechatInheritance 的
seed 半边，doc 注明 test convenience）在 src+tests 全仓零引用，也无
barrel 转出口，删除。同文件的 SeedEvent 类型与 copyEvents 仍被
SidechatInheritance 使用，保留；其余导出不动。

src/sidechat-core.ts

@@ -245,11 +245,6 @@ export function buildSidechatInheritance(events: readonly SidechatLogEvent[]): S
   return { seed, snapshot: null }
 }
 
-/** The seed half of {@link buildSidechatInheritance} (test convenience). */
-export function sidechatSeed(events: readonly SidechatLogEvent[]): SeedEvent[] {
-  return buildSidechatInheritance(events).seed
-}
-
 /**
  * Structured text snapshot of the parent's OPEN turn (from its `turn/start`
  * to the log tail): the accumulated assistant/reasoning output verbatim