Eliminate double fetch per sync cycle

[lujunsan]

Summary

• Remove CurrentHash from RegistryHandler interface — it was a broken
  abstraction that promised a lightweight hash check but performed a full
  fetch in all three implementations (git, file, API)
• IsDataChanged now calls FetchRegistry directly and returns the
  FetchResult alongside the changed/unchanged boolean
• ShouldSync threads the FetchResult through to PerformSync, which
  reuses it instead of fetching again — reducing fetches per sync cycle
  from two to one for all source types
• The fallback path (prefetched == nil) is preserved for error cases

Test plan

• Unit tests pass (task test)
• Manually verified with docker-compose using a git source:
  • First sync: one clone, prefetch reused, data stored
  • Subsequent sync, no upstream change: one clone for hash check, shouldSync:false, no store
  • Subsequent sync, different source data: one clone, prefetch reused, new hash and server count stored

FIxes #548

[codecov-commenter]

Codecov Report

❌ Patch coverage is 53.73134% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.59%. Comparing base (4ba9a30) to head (cada809).

Files with missing lines	Patch %	Lines
internal/sync/manager.go	66.66%	13 Missing and 2 partials ⚠️
internal/sync/coordinator/coordinator.go	0.00%	8 Missing ⚠️
internal/sync/mocks/mock_manager.go	0.00%	7 Missing ⚠️
internal/sync/detectors.go	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #577      +/-   ##
==========================================
+ Coverage   57.54%   57.59%   +0.05%     
==========================================
  Files          94       94              
  Lines        8460     8443      -17     
==========================================
- Hits         4868     4863       -5     
+ Misses       3261     3249      -12     
  Partials      331      331              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.