chore: move chatd and related packages to /x/ subpackage (#23445)

- Moves `coderd/chatd/`, `coderd/gitsync/`, `enterprise/coderd/chatd/`
under `x/` parent directories to signal instability
- Adds `Experimental:` glue code comments in `coderd/coderd.go`

> 🤖 This PR was created with the help of Coder Agents, and was
reviewed by my human. 🧑‍💻

coderd/x/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
+}