refactor(github): extract developer score formula into computeDeveloperScore() (JhaSourav07#1641)

## Description

Fixes JhaSourav07#297

Extracted the developer score calculation logic from
getFullDashboardData() into a reusable pure function
computeDeveloperScore().

This refactor separates scoring logic from dashboard aggregation logic,
improving readability, maintainability, and testability while preserving
the existing scoring behavior.

### What changed

- Added computeDeveloperScore() helper
- Replaced inline score calculation
- Added comprehensive unit tests
- Preserved existing behavior

### Validation performed

- Ran formatting checks
- Ran lint checks
- Ran test suite
- Verified behavior remains unchanged

## Pillar

- [ ] 🎨 Pillar 1 — New Theme Design
- [ ] 📐 Pillar 2 — Geometric SVG Improvement
- [ ] 🕐 Pillar 3 — Timezone Logic Optimization
- [x] 🛠️ Other (Bug fix, refactoring, docs)

## Visual Preview

N/A — Internal refactor with no visual changes.

## Checklist before requesting a review:

- [x] I have read the CONTRIBUTING.md file.
- [x] I have tested these changes locally.
- [x] I have run npm run format and npm run lint.
- [x] My commits follow the Conventional Commits format.
- [x] I have made sure this PR contains one logical change.
- [x] All tests pass successfully.

Author: JhaSourav07