feat: establish GitHub Actions CI pipeline

Implements a robust Continuous Integration pipeline in .github/workflows/ci.yml.

The pipeline is triggered on pull requests and pushes to the leader branch.
It uses concurrency groups to cancel redundant runs.

The pipeline includes the following steps:
- Dependency caching based on the pnpm-lock.yaml hash.
- Static analysis via 'pnpm run lint', 'tsc --noEmit', and 'pnpm run knip'.
- Code formatting verification using 'pnpm run format:check'.
- Security auditing using 'pnpm audit' to identify and block high-severity vulnerabilities.

Author: google-labs-jules[bot]