Improve image sync workflow usability

[JuckZ]

Motivation

• Make the GitHub Actions workflow more generic, configurable and robust so it can be reused across projects and environments.
• Replace repository-specific examples with safe, generic templates and document how to provide credentials via Actions secrets.

Description

• Replace the workflow .github/workflows/sync-images.yml with a configurable template that adds workflow_dispatch inputs (image_list_path, config_secret_name, image_syncer_version, dry_run), pins checkout@v4, and grants minimal contents: read permission.
• Add input validation, environment variable wiring, a resilient download step using curl --retry, and compose a runtime config.yaml by concatenating the secret content with the chosen image list before invoking ./image-syncer; support a dry_run mode that prints generated config keys.
• Rewrite README.md to explain usage, manual inputs, mapping format, troubleshooting and maintenance tips, and update example/config.yaml to a generic credential template showing where to place optional image-syncer settings.
• Replace imagelist.yaml contents with a minimal example mapping and empty historyRecord, and add .gitignore entries to avoid committing generated config.yaml and downloaded image-syncer artifacts.

Testing

• Ran git diff --check to validate whitespace and patch issues and it succeeded.
• Validated YAML parsing of imagelist.yaml, example/config.yaml and .github/workflows/sync-images.yml using the one-liner ruby -e "require 'yaml'; ['imagelist.yaml','example/config.yaml','.github/workflows/sync-images.yml'].each{|p| YAML.load_file(p); puts \"#{p} ok\"}" and all files loaded OK.
• Confirmed repository state with git status --short after the changes and a commit was created successfully.

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Codex Task