feat: show agent badge on workspace list (#23453)

- Adds `GET /api/experimental/chats/by-workspace` endpoint that returns
workspace_id → latest chat_id mapping
- Modifies FE to fetch this alongside the workspace list, gated on
`agents` experiment and render an "Agent" badge similar to the existing
"Task" badge in `WorkspacesTable`
- Badge links to the "latest chat" linked to the given workspace.

Notes:
- Intentionally uses `fetchWithPostFilter` for RBAC to decouple from
workspaces API — will migrate to `workspaces_expanded` view later.
- If users have multiple chats linked to the same workspace, the badge
will link to the most recently updated one.

> 🤖 This PR was created with the help of Coder Agents, and has been
reviewed by my human. 🧑‍💻

coderd/coderd.go

@@ -1155,6 +1155,7 @@ func New(options *Options) *API {
 				apiKeyMiddleware,
 				httpmw.RequireExperimentWithDevBypass(api.Experiments, codersdk.ExperimentAgents),
 			)
+			r.Get("/by-workspace", api.chatsByWorkspace)
 			r.Get("/", api.listChats)
 			r.Post("/", api.postChats)
 			r.Get("/models", api.listChatModels)