No portable way for a plugin's MCP server to receive the user's workspace root #40

v1 expands ${PLUGIN_ROOT}/${PLUGIN_DATA} and recommends the plugin root as the default subprocess cwd. A server whose tools operate on the user's project — writing project files, keeping project-scoped state — has no portable way to learn that path, so any such server either requires per-call absolute paths or silently writes into the plugin directory.

Concrete case: reelier serve --workspace <path> exists precisely for this, and no conformant manifest can populate it.

Proposal: an optional client-supplied ${WORKSPACE_ROOT} expansion variable (defined as "the directory the user's session is rooted in, if the client has one"), with absence meaning the client provides no workspace notion.