detect upstream-merged PRs closed by commit

[fematarazzo]

Some projects (Vim, Linux, Git) don't use GitHub's native merge button — maintainers cherry-pick changes and close the PR with a reference commit. GitHub marks these as CLOSED with mergedAt: null, so they were being counted as rejected.

This adds timelineItems to the GraphQL query to fetch the ClosedEvent.closer on each PR. If the closer is a Commit, the PR was applied upstream and is now counted as merged in stats. The merged_date is set from closedAt in those cases.

Changes:

• api/types.ts — added timelineItems field to GitHubPR
• api/utils/github-api.ts — extended query to fetch ClosedEvent.closer.__typename
• api/utils/inner_data_processor.ts — added isClosedByCommit() helper; mapPRStatus now returns merged for commit-closed PRs; processGitHubPRs uses closedAt as the merge date