Switch to upstream MCP registry format and add skills provider methods

[rdimitrov]

Summary

The registry system used a legacy format as its internal representation. The upstream MCP format already exists in toolhive-core and toolhive-catalog but was only used by the API provider path. This switches to upstream as the primary format and adds skills methods to the Provider interface.

• Remove GetImageServer/SearchImageServers/ListImageServers from Provider interface; single caller migrated to type assertion
• Switch embedded catalog from catalog.Legacy() to catalog.Upstream()
• Add dual-format auto-detection for custom registry files and URLs; legacy format emits a deprecation warning
• Add short-name resolution for backward compatibility (osv resolves to io.github.stacklok/osv; errors on ambiguity)
• Add ListAvailableSkills(), GetSkill(), and SearchSkills() to Provider interface with implementations on all providers
• Wire SkillsClient into APIRegistryProvider/CachedAPIRegistryProvider with caching for API-backed skills

Fixes #

Type of change

• Bug fix
• New feature
• Refactoring (no behavior change)
• Dependency update
• Documentation
• Other (describe):

Test plan

• Unit tests (task test)
• E2E tests (task test-e2e)
• Linting (task lint-fix)
• Manual testing (describe below)

Changes

File	Change

Does this introduce a user-facing change?

Special notes for reviewers

Large PR Justification

[Explain why this PR must be large, such as:]

• Superseded Add skills methods to registry Provider interface #4191

[codecov]

Codecov Report

❌ Patch coverage is 34.46970% with 173 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.12%. Comparing base (b65f94f) to head (c0634b7).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/registry/provider_remote.go	16.00%	37 Missing and 5 partials ⚠️
pkg/registry/provider_cached.go	0.00%	39 Missing ⚠️
pkg/registry/provider_local.go	29.09%	34 Missing and 5 partials ⚠️
pkg/registry/provider_api.go	0.00%	20 Missing ⚠️
pkg/registry/provider_base.go	61.29%	8 Missing and 4 partials ⚠️
pkg/config/registry.go	54.16%	3 Missing and 8 partials ⚠️
pkg/registry/upstream_parser.go	79.48%	4 Missing and 4 partials ⚠️
pkg/runner/retriever/retriever.go	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4171      +/-   ##
==========================================
- Coverage   69.29%   69.12%   -0.17%     
==========================================
  Files         466      467       +1     
  Lines       46774    46963     +189     
==========================================
+ Hits        32412    32464      +52     
- Misses      11874    11924      +50     
- Partials     2488     2575      +87     

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[github-actions]

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## Large PR Justification

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