Add a side placement for extension widgets, so a plugin can host a persistent panel

[jasoncarreira]

Summary

Extension widgets can only be placed inline around the editor. There is no side region — no sidebar, no
persistent panel — so an extension that wants to show durable state alongside the conversation has nowhere to put
it. I would like a side placement for extension widgets.

Environment. prime-agent 0.7.2, daemon protocol 7 (protocol-7-schema-16-1bcb9e7f1a49), build 83a0f9f-dirty, Node 24.11.1, macOS 15 (darwin 25.5.0).

Current surface

/** Placement for extension widgets. */
export type WidgetPlacement = "aboveEditor" | "belowEditor";

ExtensionUIContext is generous for transient interaction — select, confirm, input, notify,
onTerminalInput, setStatus, setWorkingMessage, setWorkingIndicator — but all of it is either modal or a
single line of status. Anything larger than a status string has to be printed into the conversation, where it
scrolls away and competes with the transcript.

What we would use it for

We orchestrate long autonomous builds. The state worth showing continuously is a handful of rows — the current
gate, each parallel slice and its attempt count, the lock owner, the last verification result. It updates every
few seconds for hours. In our opencode adapter that lives in a sidebar component and stays out of the
conversation entirely; there is no equivalent placement here, so our prime adapter ships no UI at all.

A "sidebar" (or "side") member of WidgetPlacement, with a width hint and the same render contract the
existing placements use, would be enough. It does not need to be resizable or focusable to be useful.

Relationship to #1462

#1462 asks for client-local UI extensions to
execute at all in daemon-backed interactive chats, noting that getEditorComponent() and friends are currently
no-ops. That is the execution half and I support it. This is the placement half: even once those callbacks
run, aboveEditor/belowEditor are the only places a widget can appear. Both are needed for a panel that
persists beside the conversation, and they seem independently implementable.