expose remote config loading

[rishiz27]

this exposes the existing remote config fetch as LoadRemoteRef so policy-bot can load the repo policy and shared parent separately, then combine them in a later change.

it returns the file contents without following another remote reference or falling back to an org default. it uses the same client selection, private-remotes setting, caching, cancellation and error handling we already have.

also rejects malformed repo references that could change the request URL before any installation lookup or fetch.

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• Improvement (Improving existing functionality)
• Fix (Fixing an issue with existing functionality)
• Break (Creating a new major version by breaking public APIs)
• Deprecation (Removing functionality in a non-breaking way)
• Migration (Automatically moving data/functionality to a new system)

Description

expose remote config loading

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