Added standalone yoi-workspace-server binary entrypoint with serve command and options for workspace/db/frontend/listen. Added yoi workspace serve launcher that resolves an external executable via YOI_WORKSPACE_SERVER_COMMAND or sibling yoi-workspace-server without linking the workspace-server crate into yoi. Updated package.nix to build/install both separate binaries. Validation passed: cargo fmt --check; cargo check -p yoi -p yoi-workspace-server; cargo test -p yoi workspace; cargo test -p yoi-workspace-server; cargo build -p yoi -p yoi-workspace-server; help/launch smoke tests; cargo tree dependency check; git diff --check; yoi ticket doctor; yoi objective doctor; nix build .#yoi --no-link.
