fix: allow loading registries and policies from hidden directories

[kuklyy]

Summary

Removes unnecessary is_hidden() filter that prevented loading from directories starting with . (dot). The existing file type filters are sufficient to exclude problematic paths.

Changes

• Removed is_hidden() function and filter from weaver_resolver/src/lib.rs (registry loading)
• Removed is_hidden() function and filter from weaver_checker/src/lib.rs (policy loading)
• Added integration test to verify hidden registry directories work correctly

Test plan

• Added test test_hidden_directory that verifies loading from .hidden_reg/ directory
• All existing tests pass
• CI quality checks pass (fmt, clippy, doc)

Fixes #958

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.3%. Comparing base (aea946b) to head (ab10d41).

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #992     +/-   ##
=======================================
- Coverage   78.3%   78.3%   -0.1%     
=======================================
  Files         77      77             
  Lines       6122    6114      -8     
=======================================
- Hits        4799    4789     -10     
- Misses      1323    1325      +2     

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[lquerel]

I'd prefer an opt-in parameter (e.g. --include-hidden) that allows exploring hidden files/directories, to avoid surprising other users who would not find it normal for a CLI to inspect files in locations such as .git, .svn, .idea, .vscode, .cache, .local, .venv, .aws/credentials, .kube/config, ...