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/chatd/chattool/writefile.go

@@ -0,0 +1,51 @@
+package chattool
+
+import (
+	"context"
+	"strings"
+
+	"charm.land/fantasy"
+
+	"github.com/coder/coder/v2/codersdk/workspacesdk"
+)
+
+type WriteFileOptions struct {
+	GetWorkspaceConn func(context.Context) (workspacesdk.AgentConn, error)
+}
+
+type WriteFileArgs struct {
+	Path    string `json:"path"`
+	Content string `json:"content"`
+}
+
+func WriteFile(options WriteFileOptions) fantasy.AgentTool {
+	return fantasy.NewAgentTool(
+		"write_file",
+		"Write a file to the workspace.",
+		func(ctx context.Context, args WriteFileArgs, _ fantasy.ToolCall) (fantasy.ToolResponse, error) {
+			if options.GetWorkspaceConn == nil {
+				return fantasy.NewTextErrorResponse("workspace connection resolver is not configured"), nil
+			}
+			conn, err := options.GetWorkspaceConn(ctx)
+			if err != nil {
+				return fantasy.NewTextErrorResponse(err.Error()), nil
+			}
+			return executeWriteFileTool(ctx, conn, args)
+		},
+	)
+}
+
+func executeWriteFileTool(
+	ctx context.Context,
+	conn workspacesdk.AgentConn,
+	args WriteFileArgs,
+) (fantasy.ToolResponse, error) {
+	if args.Path == "" {
+		return fantasy.NewTextErrorResponse("path is required"), nil
+	}
+
+	if err := conn.WriteFile(ctx, args.Path, strings.NewReader(args.Content)); err != nil {
+		return fantasy.NewTextErrorResponse(err.Error()), nil
+	}
+	return toolResponse(map[string]any{"ok": true}), nil
+}