feat(gitprovider): add github discussions and git commits support

[FelixTJDietrich]

Description

Adds support for GitHub Discussions and Git Commits to the gitprovider module, built on the new GraphQL-native architecture from #621.

Discussions

• Discussion entity with state, categories, Q&A support (accepted answers), and labels
• DiscussionCategory entity using String ID (node_id) since GitHub's GraphQL API doesn't expose databaseId for categories
• DiscussionComment entity with reply threading and author association tracking
• Sync service and processor following the established patterns

Git Commits

• GitCommit entity using SHA as primary key (provider-agnostic)
• GitCommitFileChange entity for tracking file changes per commit
• Support for author/committer metadata (separate from linked User entities)
• Sync service and processor for GraphQL-based fetching

GraphQL Operations

• GetRepositoryDiscussions.graphql - Fetches discussions with comments, categories, labels
• GetRepositoryCommits.graphql - Fetches commit history with statistics
• GetDiscussionCategories.graphql - Fetches available discussion categories

How to Test

1. Run ./mvnw compile to verify compilation
2. CI covers database migration and integration tests
3. Manual testing requires enabling discussion sync in a workspace

Notes

• This PR should be merged after chore(gitprovider): replace hub4j with typed graphql sync #621 lands on main
• Webhook handlers for real-time updates can be added in a follow-up PR

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