feat: add global chat workspace TTL setting (#23265)

- Add `agents_workspace_ttl` site config (default: whatever the template
says a.k.a. `0s`)
- Expose via GET/PUT `/api/experimental/chats/config/workspace-ttl`
- Chat tool reads setting and passes `TTLMillis` on workspace creation
- Existing autostop infrastructure handles the rest (zero changes to
LifecycleExecutor, CalculateAutostop, or activity bumping)
- ⚠️ Template-level `UserAutostopEnabled=false` overrides this global
default. Not touching this.
- Frontend: "Workspace Lifetime" control in /agents/settings Behavior
tab (admin-only)

> This PR was created with the help of Coder Agents, and has been
reviewed by several humans and robots. 🤖🤝🧑‍💻

coderd/coderd.go

@@ -1177,6 +1177,8 @@ func New(options *Options) *API {
 				r.Put("/desktop-enabled", api.putChatDesktopEnabled)
 				r.Get("/user-prompt", api.getUserChatCustomPrompt)
 				r.Put("/user-prompt", api.putUserChatCustomPrompt)
+				r.Get("/workspace-ttl", api.getChatWorkspaceTTL)
+				r.Put("/workspace-ttl", api.putChatWorkspaceTTL)
 			})
 			// TODO(cian): place under /api/experimental/chats/config
 			r.Route("/providers", func(r chi.Router) {