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/chatparam.go

@@ -0,0 +1,50 @@
+package httpmw
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/coder/coder/v2/coderd/database"
+	"github.com/coder/coder/v2/coderd/httpapi"
+	"github.com/coder/coder/v2/codersdk"
+)
+
+type chatParamContextKey struct{}
+
+// ChatParam returns the chat from the ExtractChatParam handler.
+func ChatParam(r *http.Request) database.Chat {
+	chat, ok := r.Context().Value(chatParamContextKey{}).(database.Chat)
+	if !ok {
+		panic("developer error: chat param middleware not provided")
+	}
+	return chat
+}
+
+// ExtractChatParam grabs a chat from the "chat" URL parameter.
+func ExtractChatParam(db database.Store) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
+			ctx := r.Context()
+			chatID, parsed := ParseUUIDParam(rw, r, "chat")
+			if !parsed {
+				return
+			}
+
+			chat, err := db.GetChatByID(ctx, chatID)
+			if httpapi.Is404Error(err) {
+				httpapi.ResourceNotFound(rw)
+				return
+			}
+			if err != nil {
+				httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+					Message: "Internal error fetching chat.",
+					Detail:  err.Error(),
+				})
+				return
+			}
+
+			ctx = context.WithValue(ctx, chatParamContextKey{}, chat)
+			next.ServeHTTP(rw, r.WithContext(ctx))
+		})
+	}
+}