feat: Rebrand to gitcaddy-runner with upload helper

- Rename binary from act_runner to gitcaddy-runner
- Update all user-facing strings (Gitea → GitCaddy)
- Add gitcaddy-upload helper with automatic retry for large files
- Add upload helper package (internal/pkg/artifact)
- Update Docker image name to marketally/gitcaddy-runner

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/upload-helper/main.go

@@ -0,0 +1,38 @@
+// Copyright 2026 MarketAlly. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+
+	"gitea.com/gitea/act_runner/internal/pkg/artifact"
+)
+
+func main() {
+	url := flag.String("url", "", "Upload URL")
+	token := flag.String("token", "", "Auth token")
+	file := flag.String("file", "", "File to upload")
+	retries := flag.Int("retries", 5, "Maximum retry attempts")
+	flag.Parse()
+
+	if *url == "" || *token == "" || *file == "" {
+		fmt.Fprintf(os.Stderr, "GitCaddy Upload Helper - Reliable file uploads with retry\n\n")
+		fmt.Fprintf(os.Stderr, "Usage: gitcaddy-upload -url URL -token TOKEN -file FILE\n\n")
+		fmt.Fprintf(os.Stderr, "Options:\n")
+		flag.PrintDefaults()
+		os.Exit(1)
+	}
+
+	helper := artifact.NewUploadHelper()
+	helper.MaxRetries = *retries
+
+	if err := helper.UploadWithRetry(*url, *token, *file); err != nil {
+		fmt.Fprintf(os.Stderr, "Upload failed: %v\n", err)
+		os.Exit(1)
+	}
+
+	fmt.Println("Upload succeeded!")
+}