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/httpmw/requestid.go

@@ -15,13 +15,24 @@ type requestIDContextKey struct{}
 
 // RequestID returns the ID of the request.
 func RequestID(r *http.Request) uuid.UUID {
-	rid, ok := r.Context().Value(requestIDContextKey{}).(uuid.UUID)
+	rid, ok := RequestIDOptional(r)
 	if !ok {
 		panic("developer error: request id middleware not provided")
 	}
 	return rid
 }
 
+// RequestIDOptional returns the request ID when present.
+func RequestIDOptional(r *http.Request) (uuid.UUID, bool) {
+	rid, ok := r.Context().Value(requestIDContextKey{}).(uuid.UUID)
+	return rid, ok
+}
+
+// WithRequestID stores a request ID in the context.
+func WithRequestID(ctx context.Context, rid uuid.UUID) context.Context {
+	return context.WithValue(ctx, requestIDContextKey{}, rid)
+}
+
 // AttachRequestID adds a request ID to each HTTP request.
 func AttachRequestID(next http.Handler) http.Handler {
 	return http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {