Add GitHub as a supported integration platform

[Copilot]

GitHub was not a supported integration platform, making it impossible for users to sync their data/predictions with a GitHub repository. All attempts to create a GitHub integration were silently rejected by validation.

Changes Made

New: src/github.js — GitHub REST API client

• syncPrediction(owner, repo, prediction) — commit prediction JSON to a repo (create or update via Contents API)
• createPredictionIssue(owner, repo, prediction) — open a formatted GitHub Issue with prediction report
• listSyncedPredictions(owner, repo) — list previously synced prediction files
• getRepository(owner, repo) — fetch repo metadata

Backend

• Integration model ENUM: added 'github'
• Route validator isIn([...]): added 'github'
• getPlatforms(): added GitHub entry with sync, version-control, repository, collaboration features
• formatPredictionForPlatform(): added 'github' case returning file path + commit message

Frontend / Config

• src/index.js: exports GitHub alongside existing clients
• Integrations.js: added GitHub to the subtitle
• .env.example: added GITHUB_TOKEN with PAT instructions
• INTEGRATIONS.md / README.md: documented GitHub platform config, required scopes, and export format

Tests

• tests/github.test.js: 15 unit tests covering constructor, syncPrediction (new file, existing file with SHA, custom path), createPredictionIssue, listSyncedPredictions, headers, network errors, non-JSON responses

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 📚 Documentation update
• 🧪 Test updates

Testing

• Unit tests pass
• Manual testing completed
• No breaking changes

Checklist

• My code follows the code style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published