Customizable context menus (user-defined entries and plugin actions)

[bigapple12]

Current behavior

The right-click context menus (pane / tab / workspace) have a fixed set of built-in items. There is no way to add entries: [[keys.command]] only binds keys, and plugin [[actions]] can only be invoked via keybindings or the CLI. The contexts field on actions suggests placement but has no visible effect on the menus.

Requested change

Make the context menus extensible, e.g.:

• show plugin actions in the matching menu based on their contexts (["pane"] → pane menu, ["workspace"] → workspace menu), and/or
• a [[menu.command]] config section like [[keys.command]] for user-defined menu entries (title + shell / plugin_action command).

Reason

Mouse-centric workflows end at the fixed menu. Concrete examples I wanted: paste from clipboard (terminals like iTerm2/Ghostty offer this in their native right-click menu, but herdr's mouse capture replaces it), open a lazygit / file-manager popup for this pane, toggle a live diff for this workspace. All of these already exist as keybindings or plugin actions — they just can't be reached from the menu.

(Originally filed as #1511; moving here per the issue-tracker policy.)