fenxin-fx/dsh-default-workspace-plugin: New sessions don't have to select a workspace first

`dsh-default-workspace` is a community-maintained DSH plugin designed to simplify the new session startup process. To address the issue where new users must manually select a workspace before chatting, this plugin reduces pre-requisites by automatically preparing a default workspace or providing an entry point to "start a session outside the workspace." Its core functions include automatically creating and registering a default workspace at `<DSH_HOME>/workspaces/default` during the first launch; adding a "no workspace" session option in the workspace selector, where the session's working directory falls back to the host's `process.cwd()` and is displayed as ungrouped. The plugin design follows the principles of zero dependencies, idempotency, non-fatal errors, and zero footprint. It does not intervene by default when workspaces already exist, unless configured with `always: true`. It supports configuration via configuration options or environment variables.