feat: support Hugging Face revision and commit pinning

[elbruno]

Context

Production model bundles must not silently change when main changes.

Proposed API

public sealed class DownloadRequest
{
    public required string RepoId { get; init; }
    public string Revision { get; init; } = "main";
    public string? ExpectedCommitSha { get; init; }
}

Requirements

• Revision may be branch, tag, or commit.
• Resolve immutable SHA when possible.
• Cache includes repo and resolved revision.
• Store resolution metadata.
• Existing callers default to main.
• CLI supports --revision.

Acceptance criteria

• Download by tag.
• Download by commit.
• Revisions do not overwrite each other.
• Result reports resolved commit.
• Existing API compatible.
• Unit tests use injectable HTTP.

Priority: P0
Labels: enhancement, reproducibility, huggingface, ElBruno.Speech