ci: implement automated benchmarking and regression report workflow in GitHub Actions

[archittmittal]

Objective & Scope

Implement automated benchmarking CI workflow in .github/workflows/benchmarks.yml.

Technical Details & Implementation Plan

• GitHub Actions workflow running automated benchmark comparisons on pull requests
• Post benchmark summary comment to PR automatically

Deliverable Files

• .github/workflows/benchmarks.yml

Acceptance Criteria & Checkpoints

• Workflow passes syntax and execution validation

Parent Epic

Part of Epic #44