feat: add experimental agents support (#22290)

feat: add AI chat system with agent tools and chat UI

Introduce the chatd subsystem and Agents UI for AI-powered chat
within Coder workspaces.

- Add chatd package with chat loop, message compaction, prompt
  management, and LLM provider integration (OpenAI, Anthropic)
- Add agent tools: create workspace, list/read templates, read/write/
  edit files, execute commands
- Add chat API endpoints with streaming, message editing, and
  durable reconnection
- Add database schema and migrations for chats, chat messages, chat
  providers, and chat model configs
- Add RBAC policies and dbauthz enforcement for chat resources
- Add Agents UI pages with conversation timeline, queued messages
  list, diff viewer, and model configuration panel
- Add comprehensive test coverage including coderd integration tests,
  chatd unit tests, and Storybook stories
- Gate feature behind experiments flag

---------

Co-authored-by: Cian Johnston <cian@coder.com>
Co-authored-by: Danielle Maywood <danielle@themaywoods.com>
Co-authored-by: Jeremy Ruppel <jeremy@coder.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

coderd/aitasks.go

@@ -192,7 +192,8 @@ func (api *API) tasksCreate(rw http.ResponseWriter, r *http.Request) {
 	})
 	defer commitAuditWS()
 
-	workspace, err := createWorkspace(ctx, aReqWS, apiKey.UserID, api, owner, createReq, r, &createWorkspaceOptions{
+	workspace, err := createWorkspace(ctx, aReqWS, apiKey.UserID, api, owner, createReq, &createWorkspaceOptions{
+		remoteAddr: r.RemoteAddr,
 		// Before creating the workspace, ensure that this task can be created.
 		preCreateInTX: func(ctx context.Context, tx database.Store) error {
 			// Create task record in the database before creating the workspace so that