feat: persist data/ to repository in GitHub Action workflow

[unhappychoice]

Summary

• Add git commit & push steps to action.yml so data/ persists between workflow runs
• Daily events accumulate correctly across multiple cron runs
• Weekly pipeline can read previously accumulated events

How it works

Daily cron (every midnight):
  checkout repo → daily-fetch → commit data/ → push

Weekly cron (Monday 9am):
  checkout repo → weekly-fetch → generate → commit data/ → push → render → deploy gh-pages

Changes

• Configure git: set bot user name/email at the start
• Commit data (daily): git add data/ && git commit && git push after daily-fetch
• Commit data (weekly): same after weekly-fetch + generate
• Idempotent: skips commit when there are no changes

Caller workflow requirement

The workflow using this action must include actions/checkout before invoking:

steps:
  - uses: actions/checkout@v4
  - uses: deariary/github-weekly-reporter@v1
    with:
      github-token: ${{ secrets.GITHUB_TOKEN }}
      mode: daily

Closes #33