feat: chat desktop backend (#23005)

Implement the backend for the desktop feature for agents.

- Adds a new `/api/experimental/chats/$id/desktop` endpoint to coderd
which exposes a VNC stream from a
[portabledesktop](https://github.com/coder/portabledesktop) process
running inside the workspace
- Adds a new `spawn_computer_use_agent` tool to chatd, which spawns a
subagent that has access to the `computer` tool which lets it interact
with the `portabledesktop` process running inside the workspace
- Adds the plumbing to make the above possible

There's a follow up frontend PR here:
https://github.com/coder/coder/pull/23006

coderd/database/queries/chats.sql

@@ -142,14 +142,16 @@ INSERT INTO chats (
     parent_chat_id,
     root_chat_id,
     last_model_config_id,
-    title
+    title,
+    mode
 ) VALUES (
     @owner_id::uuid,
     sqlc.narg('workspace_id')::uuid,
     sqlc.narg('parent_chat_id')::uuid,
     sqlc.narg('root_chat_id')::uuid,
     @last_model_config_id::uuid,
-    @title::text
+    @title::text,
+    sqlc.narg('mode')::chat_mode
 )
 RETURNING
     *;