fix: Replace exec sync command with syscall sync

[Itxaka]

• Changed the method of flushing filesystem buffers from using exec.Command("sync") to syscall.Sync().
• This change improves performance and reliability by directly invoking the system call instead of spawning a new process.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 41.43%. Comparing base (51d19f2) to head (90c05b0).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/agent/hooks/encrypt.go	0.00%	1 Missing ⚠️

❌ Your project status has failed because the head coverage (59.27%) is below the target coverage (60.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1035      +/-   ##
==========================================
- Coverage   41.48%   41.43%   -0.06%     
==========================================
  Files          62       62              
  Lines        6363     6362       -1     
==========================================
- Hits         2640     2636       -4     
- Misses       3398     3399       +1     
- Partials      325      327       +2     

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