dsh-workspace-envrc: Native direnv Integration in DSH Workspaces

`dsh-workspace-envrc` is an out-of-tree DSH plugin designed to address compatibility issues when DeepSeek Harness (DSH) handles local `.envrc` files. It fully delegates environment management to the host-installed `direnv`, avoiding state maintenance in the DSH process. Core features include applying the native `direnv` environment to Bash commands and local stdio MCP lines via `direnv exec`, as well as supporting automatic discovery and authorization verification. Installation depends on `dsh-workspace-overlay`, and configuration is defined via YAML. The plugin employs a stateless design and does not read sensitive information, but currently does not support persistent sessions.