* fix: use original title from webhook payload instead of fetched title
- Add extractOriginalTitle() helper to extract title from webhook payload
- Add originalTitle parameter to fetchGitHubData()
- Update tag mode to pass original title from webhook context
- Add tests for extractOriginalTitle and originalTitle parameter
This ensures the title used in prompts is the one that existed when the
trigger event occurred, rather than a potentially modified title fetched
later via GraphQL.
* fix: add title sanitization and explicit TOCTOU test
- Apply sanitizeContent() to titles in formatContext() for defense-in-depth
- Add explicit test documenting TOCTOU prevention for title handling
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