feat(site): include chatId in editor deep links (#23214)

## Summary

- include the current agent chat ID in VS Code and Cursor deep links
opened from the agent detail page
- extend `getVSCodeHref` so `chatId` is added only when provided
- add focused tests for deep-link generation with and without `chatId`

## Testing

- `pnpm -C site run format -- src/modules/apps/apps.ts
src/modules/apps/apps.test.ts src/pages/AgentsPage/AgentDetail.tsx`
- `pnpm -C site run check -- src/modules/apps/apps.ts
src/modules/apps/apps.test.ts src/pages/AgentsPage/AgentDetail.tsx`
- `pnpm -C site exec vitest run src/modules/apps/apps.test.ts`
- `pnpm -C site run lint:types`

---
_Generated with [`mux`](https://github.com/coder/mux) • Model:
`openai:gpt-5.4` • Thinking: `high`_

site/src/modules/apps/apps.test.ts

@@ -3,7 +3,50 @@ import {
 	MockWorkspaceAgent,
 	MockWorkspaceApp,
 } from "testHelpers/entities";
-import { getAppHref, SESSION_TOKEN_PLACEHOLDER } from "./apps";
+import { getAppHref, getVSCodeHref, SESSION_TOKEN_PLACEHOLDER } from "./apps";
+
+describe("getVSCodeHref", () => {
+	it("includes the chat ID when provided", () => {
+		const folder = "/workspace/test";
+		const href = getVSCodeHref("vscode", {
+			owner: MockWorkspace.owner_name,
+			workspace: MockWorkspace.name,
+			token: "user-session-token",
+			agent: MockWorkspaceAgent.name,
+			folder,
+			chatId: "chat-123",
+		});
+		const query = new URLSearchParams({
+			owner: MockWorkspace.owner_name,
+			workspace: MockWorkspace.name,
+			url: location.origin,
+			token: "user-session-token",
+			openRecent: "true",
+			agent: MockWorkspaceAgent.name,
+			folder,
+			chatId: "chat-123",
+		});
+
+		expect(href).toBe(`vscode://coder.coder-remote/open?${query}`);
+	});
+
+	it("omits the chat ID when none is provided", () => {
+		const href = getVSCodeHref("cursor", {
+			owner: MockWorkspace.owner_name,
+			workspace: MockWorkspace.name,
+			token: "user-session-token",
+		});
+		const query = new URLSearchParams({
+			owner: MockWorkspace.owner_name,
+			workspace: MockWorkspace.name,
+			url: location.origin,
+			token: "user-session-token",
+			openRecent: "true",
+		});
+
+		expect(href).toBe(`cursor://coder.coder-remote/open?${query}`);
+	});
+});
 
 describe("getAppHref", () => {
 	it("returns the URL without changes when external app has regular URL", () => {

site/src/modules/apps/apps.ts

@@ -34,11 +34,12 @@ type GetVSCodeHrefParams = {
 	token: string;
 	agent?: string;
 	folder?: string;
+	chatId?: string;
 };
 
 export const getVSCodeHref = (
 	app: "vscode" | "vscode-insiders" | "cursor",
-	{ owner, workspace, token, agent, folder }: GetVSCodeHrefParams,
+	{ owner, workspace, token, agent, folder, chatId }: GetVSCodeHrefParams,
 ) => {
 	const query = new URLSearchParams({
 		owner,
@@ -53,6 +54,9 @@ export const getVSCodeHref = (
 	if (folder) {
 		query.set("folder", folder);
 	}
+	if (chatId) {
+		query.set("chatId", chatId);
+	}
 	return `${app}://coder.coder-remote/open?${query}`;
 };
 

site/src/pages/AgentsPage/AgentDetail.tsx

@@ -781,6 +781,7 @@ const AgentDetail: FC = () => {
 					token: key,
 					agent: workspaceAgent.name,
 					folder: workspaceAgent.expanded_directory,
+					chatId: agentId,
 				});
 			},
 			onError: () => {