[idea] Plugin popups: centre them like herdr's dialogs, and give plugins a text field

[putnap]

idea / problem

I moved three plugin flows to popups: a workspace picker, a new-worktree dialog, and a rename for the focused agent's session. They're much better than overlay panes, but they still look out of place next to herdr's own dialogs.

• herdr's rename tab dialog opens in the centre of the window. A plugin popup opens in the centre of the panes, right of the sidebar. With my sidebar, the two are half a sidebar apart.
• Plugins have no text field. To make my rename popup look like rename tab, I run fzf with no items, fill in the current name, bind ctrl-c to clear it, and paint herdr's buttons into fzf's header with colours sampled from a screenshot. The field still comes out two rows tall.
• herdr plugin pane open --placement doesn't offer popup, so a popup can only come from the manifest.

requested change

• Centre plugin popups on the window like built-in dialogs, or let the plugin choose.
• A built-in text input dialog a plugin can open, with rename tab's look and keys (enter saves, ctrl-c clears, esc cancels), that returns the text.
• popup as a --placement value.

Related: #5715, keeping a popup open after its command exits.

why you want this

Opening my rename popup right after herdr's rename tab, the dialog jumps sideways and looks different, so it feels like a different app. A shared text field would also save every plugin author from rebuilding one in fzf or gum.