Add github_user_contribution_stats table for ContributionsCollection

[salekseev]

Problem

There is currently no table that exposes the GitHub GraphQL ContributionsCollection (contribution calendar + summary counts) for a user. This makes it difficult to query contribution summaries or build dashboards that mirror a user's contribution graph.

Proposed Solution

Add a new table (tentatively github_user_contribution_stats) that queries user(login: $login) { contributionsCollection(from: $from, to: $to) { ... } } and exposes:

• Total counts (commits, issues, PRs, PR reviews, repositories with contributed commits)
• contributionCalendar (weeks/days)
• commitContributionsByRepository (per-repo breakdown)

Notes

I have a fork and can implement this if maintainers agree with the table shape and column list.