Add a sidebar placement for plugin panes

I want a plugin pane that docks in the sidebar, below the agents list, and stays there across every workspace and tab.

The five placements today don't cover it:

• overlay and popup are temporary. I have to ask for them every time.
• split, tab and zoomed are normal panes, so they live in exactly one tab. To cover my 37 tabs I'd have to open 37 of them, each running its own process and its own API polling.

What I'm rendering is OmniRoute provider quota rings and a cache hit rate. It's glanceable status, so it belongs beside the agent list, not in the work area.

A second, smaller thing I hit on the way here. My manifest had placement = "sidebar", which 0.9.1 rejects:

Error: unknown variant `sidebar`, expected one of `overlay`, `popup`, `split`, `tab`, `zoomed`

That one bad value made herdr plugin list fail completely. All 14 of my installed plugins vanished from the output, and nothing said which manifest caused it. Skipping the bad plugin and naming it would have saved me a while.