feat(errs): export Git command classification policy

[sbalabanov]

Summary

Intent:

• Let local Git and remote Git execution backends share one retryability policy.
• Avoid requiring non-process backends to manufacture gitexec.CommandError values.

Changes:

• Export a backend-neutral CommandFailure contract accepted by the Git classifier.
• Export direct command classification and decouple the policy from platform/git/exec.
• Cover local and remote error implementations and document the extension point.

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Test Plan

Issues

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