Adds RFC 3339 timestamp filtering parameters to enable time-based issue queries.
This supports the use case of "show me recent issues and commits to analyze project progress and development efficiency".
Changes:
- Add Since and Before fields to ListRepoIssuesParams (*string type)
- Update JSON schema definition with date-time format validation
- Implement time parsing and validation in Handler method
- Pass parsed timestamps to Forgejo SDK ListIssueOption
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Co-Authored-By: Claude <noreply@anthropic.com>