Function collect_release_pr_evidence 

pub fn collect_release_pr_evidence(
    client: &GitHubClient,
    owner: &str,
    repo: &str,
    base_tag: &str,
    head_tag: &str,
) -> Result<EvidenceBundle>

Phase 1: Collect PR and commit evidence for a release range.

Resolves commits between tags, maps them to PRs via GraphQL, and fetches full PR data (reviews, files, check runs). Returns a bundle with change_requests, promotion_batches, check_runs, and build_platform populated. Other fields (repository_posture, artifact_attestations) are left as defaults for subsequent phases.