Add CI workflow for PR Validation

[rangampallysonali]

Adds a GitHub Actions CI workflow that runs on pull requests to main.

Changes:

• Builds and tests Go services
• Lints and type-checks the Python SkyPilot Bridge
• Builds the React frontend
• Validates Docker image builds without pushing images

[rangampallysonali]

Hi! I added the CI workflow for PR validation.

Since this PR introduces a new GitHub Actions workflow from a fork, the checks may require maintainer approval before they can run.