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/dump.sql

@@ -278,6 +278,10 @@ CREATE TYPE chat_message_visibility AS ENUM (
     'both'
 );
 
+CREATE TYPE chat_mode AS ENUM (
+    'computer_use'
+);
+
 CREATE TYPE chat_status AS ENUM (
     'waiting',
     'pending',
@@ -1306,7 +1310,8 @@ CREATE TABLE chats (
     root_chat_id uuid,
     last_model_config_id uuid NOT NULL,
     archived boolean DEFAULT false NOT NULL,
-    last_error text
+    last_error text,
+    mode chat_mode
 );
 
 CREATE TABLE connection_logs (