Delegate images keychain to toolhive-core

[JAORMX]

Summary

Wave 2 adoption: pkg/container/images/keychain.go (env + composite registry keychains) graduated to toolhive-core/container/images in v0.0.37 (stacklok/toolhive-core#206). This PR reduces the local file to a deprecated thin wrapper delegating to core, so the two call sites (pkg/runner/retriever, pkg/container/docker) keep compiling unchanged; the wrapper can be removed in the cleanup wave.

RegistryImageManager stays in toolhive — it is coupled to the moby client end to end (daemon.Write/daemon.Image/ImagePull/ImageBuild) and was deliberately not moved.

Bumps toolhive-core to v0.0.37.

Test plan

• task build, task lint-fix — clean
• go test ./pkg/container/... ./pkg/runner/retriever/... — pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.64%. Comparing base (32f75d9) to head (1823e32).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6147      +/-   ##
==========================================
+ Coverage   72.61%   72.64%   +0.02%     
==========================================
  Files         736      736              
  Lines       76340    76313      -27     
==========================================
+ Hits        55431    55434       +3     
+ Misses      16973    16941      -32     
- Partials     3936     3938       +2     

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