feat: add OopsError.Layers() to inspect individual error chain layers

[samber]

Closes #34 & #82

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.74%. Comparing base (3b4e15c) to head (4a7141f).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
error.go	87.50%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #110      +/-   ##
==========================================
- Coverage   88.80%   88.74%   -0.06%     
==========================================
  Files          15       15              
  Lines        1125     1155      +30     
==========================================
+ Hits          999     1025      +26     
- Misses        101      103       +2     
- Partials       25       27       +2     

Flag	Coverage Δ
unittests	88.74% <87.50%> (-0.06%)	⬇️

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[Copilot]

Pull request overview

Adds a new OopsError.Layers() API intended to let callers enumerate the error chain’s OopsError wrapping layers for per-layer inspection (in response to Issue #34’s request for top-layer access).

Changes:

• Documented err.Layers() in the README “Builder method / Getter” table.
• Added OopsError.Layers() []*OopsError to collect chain layers (outermost → innermost).
• Added unit tests covering basic layering behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
README.md	Documents the new Layers() getter.
error.go	Implements OopsError.Layers() using existing chain traversal helper.
error_test.go	Adds TestLayers for single/multi-layer chains and skipping plain-root errors.

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