feat(workspace): make workspace location configurable via env var

- Add MYCLAUDE_WORKSPACE_ROOT env var, default ~/workspace
- Auto-create workspace directory if it does not exist
- Move workspace/CLAUDE.md + .gitignore to workspace_template/
- Remove hardcoded symlink paths from workspace template
- Update get_workspace_root() with env-var priority logic
- Update metabot bots.json defaultWorkingDirectory to ~/workspace

workspace_template/.gitignore

@@ -0,0 +1,5 @@
+# 临时目录 — 禁止跟踪临时产物
+tmp/
+
+# macOS
+.DS_Store