feat: add GitHub Gmail event synchronizer v0.1

[safal207]

Adds a manual, dry-run-first synchronizer for Gmail GitHub notifications.

Included:

• JSONL or Gmail input
• GitHub comment verification
• stable event-key deduplication
• Markdown and JSONL reports
• explicit apply mode for verified journal entries only
• manual GitHub Actions workflow
• example fixture, runbook, and 8 passing unit tests

Safety boundary: Gmail detects, GitHub verifies, and only verified events may change the journal. The tool never sends comments or reactions.

[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/github-gmail-sync-v0.1

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