Add Comprehensive JSDoc to SyncService

To improve the long-term maintainability and clarity of the SyncService, all private methods should be documented with comprehensive JSDoc blocks.

Acceptance Criteria

1. Comprehensive JSDoc blocks are added to all private methods in SyncService.mjs (e.g., #ghCommand, #formatIssueMarkdown, #getIssuePath, etc.).
2. Each comment clearly describes the method's purpose, all of its parameters (@param), and its return value (@returns).
3. Methods that can throw errors are documented with the @throws tag.
4. At least one complex method, such as #getIssuePath, includes an @example block demonstrating its usage in different scenarios.