Herdr cannot put plugin items in the sidebar right-click menu

Verified against stable 0.9.3 source. A plugin cannot add an item to the workspace / tab / pane right-click menu, and no manifest field is waiting to be filled in:

• src/client/shell/context_menu.rs builds the menu as a match over the target kind returning hardcoded ClientContextMenuAction variants. There is no plugin branch.
• PluginActionContext — the enum behind contexts = ["workspace"] — is deserialized by the manifest loader, stored in PluginActionInfo, and has no consumer anywhere in src/. Declaring it is honest intent that Herdr parses and never acts on.
• The manifest accepts only build, startup, actions, events, panes and link_handlers entries. No menu or grouping key exists, and the plugins doc states native non-terminal plugin UI and runtime action registration are not part of plugin v1.

What a plugin can do for a workspace: an action with contexts = ["workspace"] (listed under plugin actions, invoked by keybinding or herdr plugin action invoke), a panes popup, and workspace display tokens via herdr workspace report-metadata --source <plugin> --token k=v, which Herdr renders beside the Space label. The invocation context arrives in HERDR_PLUGIN_CONTEXT_JSON with HERDR_WORKSPACE_ID as fallback.

Why: "add this to the right-click menu" is not implementable, and time spent looking for the manifest key is wasted. How to apply: ship the workspace-scoped action plus a popup, and say plainly in the docs that it is not in the right-click menu.