Add README Actions badge and restore the referenced CI workflow

[Copilot]

The repository referenced a GitHub Actions badge for actions/workflows/main.yml, but the target workflow file was not a valid workflow and the README did not expose the badge in Markdown. This change restores a valid main.yml workflow and adds the corresponding status badge to the README.

• README

  • Added a GitHub Actions status badge at the top of README.md
  • Points to the repository’s main.yml workflow page

• GitHub Actions

  • Replaced the invalid .github/workflows/main.yml contents with a working CI workflow
  • Configured the workflow to run on the repository’s default branch, Main
  • Scoped workflow token permissions to read-only access

• Result

  • The badge target now resolves to an actual workflow
  • Repository documentation can display current CI status from the intended workflow

[![CI](https://github.com/phuquoc81/Fastcash/actions/workflows/main.yml/badge.svg)](https://github.com/phuquoc81/Fastcash/actions/workflows/main.yml)

• Screenshot
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