docs(tasks): make one-task-per-file the default task mode (H-409)

- docs/tasks/README.md: default mode for single and multi agent, drop switch narrative
- docs/06-tasks.md: demote to read-only roadmap (phases, milestones, backlog, suggested split list without status)
- progress.md: optional archive / project-level event log; execution records live in task files
- current-state.md: project-level snapshot; task status authoritative in task frontmatter
- sync all referencing docs (start-here, coding-rules, READMEs, adoption/clean-state checklists, method-map, evaluator-rubric)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/tasks/_template.md

@@ -26,4 +26,4 @@ created: 【日期】
 ## 执行记录
 
 （做完在此记录：改了哪些文件、跑的验证命令与结果、阻塞、关键决策。
-取代逐任务追加 `progress.md`——执行记录只写进本任务文件，避免多 agent 抢改共享文件。）
+执行记录只写进本任务文件，不逐任务追加共享的 `progress.md`，避免多 agent 抢改共享文件。）