Add skills methods to registry Provider interface

[JAORMX]

Summary

• PR Add Skills API client for registry extension #4173 added SkillsClient for querying the registry's skills extension API, but it wasn't wired into the provider layer. Callers had no way to discover skills through the standard Provider interface used for servers.
• Extend the Provider interface with GetSkill, ListSkills, and SearchSkills methods. Add default empty implementations on BaseProvider (inherited by Local/Remote providers), real implementations on APIRegistryProvider that delegate to the SkillsClient, and let CachedAPIRegistryProvider inherit via embedding (uncached pass-through).

Type of change

• New feature

Test plan

• Unit tests (task test)
• Linting (task lint-fix)
• Mock regeneration (task gen)

Changes

File	Change
pkg/registry/provider.go	Add GetSkill, ListSkills, SearchSkills to Provider interface
pkg/registry/provider_base.go	Default no-op implementations returning nil/empty results
pkg/registry/provider_api.go	Add skillsClient field, create in constructor, implement skills methods with timeouts
pkg/registry/provider_cached.go	Document that skills methods are uncached pass-through
pkg/registry/mocks/mock_provider.go	Regenerated with new methods
pkg/registry/skills_provider_test.go	Tests for BaseProvider, LocalRegistryProvider, and APIRegistryProvider skills methods

Special notes for reviewers

• No caching for skills in this PR: CachedAPIRegistryProvider inherits skills methods from APIRegistryProvider via embedding — every call hits the API. Skills caching can be added in a follow-up.
• Empty results, not errors, for unsupported providers: BaseProvider returns (nil, nil) for GetSkill and empty SkillsListResult for list/search. This means Local/Remote providers silently return "no skills" rather than erroring, so callers don't need special handling per provider type.
• Timeouts match existing patterns: 10s for single lookups (GetSkill, SearchSkills), 60s for list operations (ListSkills).

Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 77.27273% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.28%. Comparing base (ca7f127) to head (7d191e6).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/registry/provider_api.go	75.00%	1 Missing and 3 partials ⚠️
pkg/registry/provider_base.go	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4191      +/-   ##
==========================================
- Coverage   69.30%   69.28%   -0.03%     
==========================================
  Files         466      466              
  Lines       46774    46798      +24     
==========================================
+ Hits        32416    32422       +6     
+ Misses      11871    11843      -28     
- Partials     2487     2533      +46     

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