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

docs/manifest.json

@@ -1410,6 +1410,10 @@
 							"title": "Builds",
 							"path": "./reference/api/builds.md"
 						},
+						{
+							"title": "Chats",
+							"path": "./reference/api/chats.md"
+						},
 						{
 							"title": "Debug",
 							"path": "./reference/api/debug.md"

docs/reference/api/chats.md

@@ -0,0 +1,45 @@
+# Chats
+
+## Archive a chat
+
+### Code samples
+
+```shell
+# Example request using curl
+curl -X POST http://coder-server:8080/api/v2/chats/{chat}/archive
+
+```
+
+`POST /chats/{chat}/archive`
+
+### Responses
+
+| Status | Meaning                                                         | Description | Schema |
+|--------|-----------------------------------------------------------------|-------------|--------|
+| 204    | [No Content](https://tools.ietf.org/html/rfc7231#section-6.3.5) | No Content  |        |
+
+## Watch chat desktop
+
+### Code samples
+
+```shell
+# Example request using curl
+curl -X GET http://coder-server:8080/api/v2/chats/{chat}/desktop \
+  -H 'Coder-Session-Token: API_KEY'
+```
+
+`GET /chats/{chat}/desktop`
+
+### Parameters
+
+| Name   | In   | Type         | Required | Description |
+|--------|------|--------------|----------|-------------|
+| `chat` | path | string(uuid) | true     | Chat ID     |
+
+### Responses
+
+| Status | Meaning                                                                  | Description         | Schema |
+|--------|--------------------------------------------------------------------------|---------------------|--------|
+| 101    | [Switching Protocols](https://tools.ietf.org/html/rfc7231#section-6.2.2) | Switching Protocols |        |
+
+To perform this operation, you must be authenticated. [Learn more](authentication.md).