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>

site/e2e/helpers.ts

@@ -1042,7 +1042,22 @@ const fillParameters = async (
 			case "number":
 				{
 					const parameterField = parameterLabel.locator("input");
-					await parameterField.fill(buildParameter.value);
+					// Dynamic parameters can hydrate after initial render and
+					// overwrite an early fill. Re-apply until the desired value
+					// is stable.
+					for (let attempt = 0; attempt < 3; attempt++) {
+						await parameterField.fill(buildParameter.value);
+						try {
+							await expect(parameterField).toHaveValue(buildParameter.value, {
+								timeout: 1000,
+							});
+							break;
+						} catch (error) {
+							if (attempt === 2) {
+								throw error;
+							}
+						}
+					}
 				}
 				break;
 			default: