perf(fs): 1MiB stream chunks, full sha256 via sha256sum subprocess, walk cap

- copy streams use 1MiB highWaterMark
- forced full sha256 uses Deno.Command(sha256sum), falls back to in-process stream;
  method recorded as hashVia and counted
- per-file organize.transfer timing log
- ORGANIZE_WALK_MAX_FILES caps manual path walks; dryRun returns truncated

Co-Authored-By: Claude Opus 5.5 <[email protected]>

packages/contracts/src/organization.ts

@@ -292,6 +292,8 @@ export const OrganizeAccepted = z.object({
   recordId: Id.nullable(),
   taskId: Id,
   planId: Id.nullable(),
+  /** dryRun 按路径遍历时文件数超过上限(ORGANIZE_WALK_MAX_FILES),计划只含前 N 个文件 */
+  truncated: z.boolean().optional(),
 });
 export type OrganizeAccepted = z.infer<typeof OrganizeAccepted>;
 
@@ -558,6 +560,8 @@ export type Dashboard = z.infer<typeof Dashboard>;
  */
 export const OrganizePlanArtifactContent = z.object({
   request: z.record(z.string(), z.unknown()).optional(),
+  /** 源目录文件数超过上限,计划只含前 N 个文件 */
+  truncated: z.boolean().optional(),
   status: z.string().optional(),
   reason: z.string().nullable().optional(),
   plan: z