Fork go-containerregistry as local submodule

[Copilot]

Replaces the external github.com/google/go-containerregistry dependency with a local fork to enable independent modifications.

Changes

• Added submodule: third_party/go-containerregistry at v0.20.6 (commit 59a4b859)
• go.mod: Added replace directive pointing to local fork
• go.sum: Removed remote package checksums

Implementation

The Go module replace directive transparently redirects all imports:

// In go.mod
replace github.com/google/go-containerregistry => ./third_party/go-containerregistry

No code changes required - 38 files importing github.com/google/go-containerregistry/* now use the local fork without modification.

Original prompt

We need to fork go-containerregistry in this codebase, we will need to alter it in future, we should not use this version anywhere:
github.com/google/go-containerregistry

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