feat: conclude manifest-sync experiment

[srtaalej]

Summary

• Removes the manifest-sync experiment flag and enables two-way manifest sync by default
• Unhides slack manifest sync command
• Replaces the old confirm prompt flow in shouldUpdateManifest with a call to manifest.Sync
• Updates experiment docs

Test plan

• make lint passes
• make test passes
• Manual test: slack manifest sync works without --experiment manifest-sync
• Manual test: slack deploy triggers sync on manifest mismatch

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.23%. Comparing base (9582a26) to head (156617e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #624      +/-   ##
==========================================
- Coverage   72.28%   72.23%   -0.05%     
==========================================
  Files         238      238              
  Lines       20087    20036      -51     
==========================================
- Hits        14519    14474      -45     
+ Misses       4299     4294       -5     
+ Partials     1269     1268       -1     

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