[idea] feat: plugin sidebar slots — persistent sidebar entries for plugins

[xxdesmus]

idea / problem

The sidebar is the always-visible control surface in Herdr — it's where users see what's running and navigate between workspaces and agents. Plugins can open panes, register actions, and respond to events, but they have no way to put anything in the sidebar. Every plugin-surfaced pane has to be invoked on-demand (keybind, command palette) and disappears from view the moment it loses focus. There's no persistent entry point.

Full disclosure: Claude Code helped me write this. I am no programmer.

requested change

A mechanism for plugins to register a clickable entry in the sidebar. A few possible shapes — I am not attached to any of them and defer to what fits the architecture:

1. A [[sidebar]] manifest section with a label and an action to invoke on click.
2. A contexts = ["sidebar"] field on an existing [[actions]] or [[panes]] entry that opts it into sidebar display.
3. A dedicated section below the agent panel that lists installed plugins declaring a pane entry, each clickable to open.

why you want this

I built a plugin that browses OpenCode AI coding sessions from their local SQLite database. It shows all sessions with live status (running/waiting/idle/done), project, cost, token usage, and lets you resume any session in a new pane. It polls Herdr's agent.list API every few seconds so the status stays current.

The pane itself works well. What's missing is that there's nowhere to put it that feels permanent. Agents get sidebar presence by default — users can always see what's running, what's blocked, and jump to it. A plugin pane that does the same kind of job (show you what's happening, let you act on it) should be able to opt into that same pattern. Right now it's hidden behind a keybind users have to remember.

[adriangsguruwalk]

I like this Idea as well. I think allowing to extend certain parts of the UI is really powerfull, provided herdr provides only the building blocks and the user hack-it the way she/he likes. I can also work on this if you want/need it