Closes #747.

Summary

EN:

• Fixed onboarding fresh-config fallback so it respects NULLCLAW_HOME and NULLCLAW_WORKSPACE.
• Prevented Docker Compose onboarding from drifting to HOME/.nullclaw/workspace when the config file is not yet present.
• Added a regression test covering the container env-var fallback used by the official image.
• Updated Docker / Docker Compose installation docs in both English and Chinese to call out the default workspace path.

ZH:

• 修复了 onboarding 在 fresh-config 回退路径中忽略 NULLCLAW_HOME 和 NULLCLAW_WORKSPACE 的问题。
• 避免 Docker Compose 初始化在首次运行且配置文件不存在时错误回退到 HOME/.nullclaw/workspace。
• 新增回归测试，覆盖官方镜像使用的环境变量路径回退逻辑。
• 同步更新中英文 Docker / Docker Compose 安装文档，明确默认 workspace 路径。

Validation

• zig build test --summary all: 6128 passed, 11 skipped, 0 failures.
• zig build -Doptimize=ReleaseSmall: success.
• Manual verification: not run in this environment; the Docker onboarding fallback path is covered by the new regression test.

Notes

• Keeps onboarding path resolution consistent with Config.load() and the official container env defaults.