Add default repository prefix option

[Haihan-Jiang]

Adds name.WithDefaultRepositoryPrefix so callers can choose the default prefix used for single-component repository names.

The existing behavior stays the same unless the option is set:

• ubuntu still defaults to index.docker.io/library/ubuntu:latest
• WithDefaultRegistry(...) still defaults only the registry/tag unless a repository prefix is explicitly provided
• WithDefaultRepositoryPrefix("base") makes ubuntu parse as index.docker.io/base/ubuntu:latest

Fixes #2145.

Tests:

• go test ./pkg/name
• go test ./pkg/name ./pkg/v1/remote/transport
• git diff --check

[Haihan-Jiang]

This is ready for review. The default repository prefix option is added, and the visible checks are green.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.10345% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.97%. Comparing base (31df54c) to head (a0dd82c).
⚠️ Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/name/repository.go	90.90%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2326      +/-   ##
==========================================
+ Coverage   56.91%   56.97%   +0.05%     
==========================================
  Files         165      165              
  Lines       11362    11382      +20     
==========================================
+ Hits         6467     6485      +18     
- Misses       4134     4135       +1     
- Partials      761      762       +1     

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