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/pubsub/chatevent.go

@@ -0,0 +1,46 @@
+package pubsub
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	"github.com/google/uuid"
+	"golang.org/x/xerrors"
+
+	"github.com/coder/coder/v2/codersdk"
+)
+
+func ChatEventChannel(ownerID uuid.UUID) string {
+	return fmt.Sprintf("chat:owner:%s", ownerID)
+}
+
+func HandleChatEvent(cb func(ctx context.Context, payload ChatEvent, err error)) func(ctx context.Context, message []byte, err error) {
+	return func(ctx context.Context, message []byte, err error) {
+		if err != nil {
+			cb(ctx, ChatEvent{}, xerrors.Errorf("chat event pubsub: %w", err))
+			return
+		}
+		var payload ChatEvent
+		if err := json.Unmarshal(message, &payload); err != nil {
+			cb(ctx, ChatEvent{}, xerrors.Errorf("unmarshal chat event"))
+			return
+		}
+
+		cb(ctx, payload, err)
+	}
+}
+
+type ChatEvent struct {
+	Kind ChatEventKind `json:"kind"`
+	Chat codersdk.Chat `json:"chat"`
+}
+
+type ChatEventKind string
+
+const (
+	ChatEventKindStatusChange ChatEventKind = "status_change"
+	ChatEventKindTitleChange  ChatEventKind = "title_change"
+	ChatEventKindCreated      ChatEventKind = "created"
+	ChatEventKindDeleted      ChatEventKind = "deleted"
+)