chore: untrack .env and ignore Autoflow personal layer

- Stop tracking .env (was committed in 31f61b5 with plaintext MinIO
  secrets; existing history cleanup is tracked as Risk-A in Phase 1
  backlog and will be addressed before launch)
- Ignore .autoflow/ entirely (per-branch personal notes layer)
- Ignore two local-only reference assets: drawio architecture export
  and toolchain manual PDF

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.env

@@ -1,2 +0,0 @@
-# Local default. start.sh will override if needed.
-USE_PREBUILD=/workspaces/model_converter_web/toolchain/prebuild

.gitignore

@@ -80,4 +80,8 @@ CLAUDE.md.backup
 
 # Autoflow Agent（由 autoflow-agent init 自動產生）
 .claude/
-.autoflow/CLAUDE.md.backup.*
+.autoflow/
+
+# 兩個本地參考資料，不進 repo
+Model Converter with Task Queue.drawio.png
+tool chain manual.pdf