Add capture-dir hook

[shunichironomura]

Summary

• Add capture-dir hook for copying, moving, or hashing directory trees
• Register the hook in the built-in pre/post registries
• Add tests and documentation for the new hook

Tests

• just lint
• just test

Closes #1057

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.33333% with 41 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.92%. Comparing base (78dc65d) to head (a30318c).

Files with missing lines	Patch %	Lines
crates/capsula-capture-dir/src/lib.rs	81.44%	41 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1059      +/-   ##
==========================================
+ Coverage   48.74%   50.92%   +2.18%     
==========================================
  Files          38       41       +3     
  Lines        3787     4033     +246     
==========================================
+ Hits         1846     2054     +208     
- Misses       1941     1979      +38     

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[shunichironomura]

Need to check the behavior of capsula push