Add CI badge to README.md for build status visibility

[Copilot]

This PR adds a GitHub Actions CI status badge to the README.md file to display the current build status of the project.

Changes Made

• Added CI badge using the standard GitHub Actions format: ![CI](https://github.com/mahata/mlack/workflows/CI/badge.svg)
• Placed the badge prominently at the top of the README, right after the main title
• Maintained proper markdown formatting with appropriate spacing

Why This Change?

The CI badge provides immediate visibility into the project's build status, helping:

• Contributors quickly see if their changes break the build
• Users understand the current health of the project
• Maintainers monitor CI status at a glance

Visual Impact

The badge now appears at the top of the README:

# mlack

![CI](https://github.com/mahata/mlack/workflows/CI/badge.svg)

It's an experiment to create a slack-like app just by vibe coding with GitHub Copilot.

The badge will automatically update to reflect the status of the "CI" workflow defined in .github/workflows/ci.yml, showing green for passing builds and red for failing ones.

Testing

• ✅ All existing tests continue to pass
• ✅ Linting passes without issues
• ✅ Build completes successfully
• ✅ No functional changes to the codebase

Created from VS Code via the GitHub Pull Request extension.

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