feat: add CI pipeline with GitHub Actions

[hirotask]

Summary

Add a CI pipeline using GitHub Actions with the following three jobs:

• lint: Code quality check with ESLint and format check
• build: Build all packages
• test: Run unit tests with Vitest

PR Checklist

• The commit message follows our guidelines: https://github.com/hirotask/flagly/blob/develop/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is resolved in this PR?

• resolves Add CI pipeline with GitHub Actions #8

Does this PR introduce a breaking change?

• Yes
• No

Other information

Added .node-version file to specify Node.js version 25.0.0.