feat(compaction): wire TaskHistory into BackgroundManager and compaction pipeline

Records task history at 6 status transitions (pending, running×2, error,
cancelled, completed). Exports TaskHistory from background-agent barrel.
Passes backgroundManager and sessionID through compaction hook chain.

src/index.ts

@@ -82,7 +82,7 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
       if (!hooks.compactionContextInjector) {
         return
       }
-      output.context.push(hooks.compactionContextInjector())
+      output.context.push(hooks.compactionContextInjector(_input.sessionID))
     },
   }
 }