Improve Code Health of the AI corpus synchronizer

[woksin]

Changed

• The AI corpus synchronizer is restructured into smaller, focused units for maintainability, with no change in behaviour (Improve Code Health of the remaining cli hotspot source files #326)

[woksin]

Reviewer notes

Behaviour-preserving refactor of a CodeScene hotspot (project 85913), batch 2. Tests, docs and config are untouched.

• Code Health: AiCorpusSynchronizer.cs 5.42 -> 9.09; new files 9.38-10.
• Local Tier 1 (CI mirror) passed on the merged tree. Docker/packaging steps run in CI only.
• Full cross-provider review passed. A point it raised about deferred enumeration of a caller-supplied rule list was then made strictly equivalent to main, and that fix was reviewed separately (passed).