refactor: reduce redundancy, remove dead code, improve structure
- Fix CodeWhisperer SetModel cache invalidation bug
- Remove dead code: gemini stub, orphaned comments, kiroMetadataEvent
- Extract paths.NewUUID() replacing 3 duplicate UUID implementations
- Export toolsrv.BuildToolListing, deduplicate tool listing render
- Extract baseBackend struct (Name/Model/SetModel) embedded in all backends
- Unify backend factory post-processing (7 if-blocks → 1)
- Split fs/handlers.go (888 lines) into handlers_{root,session,agent}.go
- Strip dead filesystem-backed tree code from fsedsl (WithReadOnly,
WithIndex, WithResolver, ValidateReadMode, resolve/writePath fallbacks)
9c90cb8954