Add GitHub PR URL verification for open source grant evidence

[teethaking]

Extend grantstream-verifier to validate GitHub PR URLs submitted as milestone evidence. Use the GitHub REST API via reqwest to check that the PR exists, is merged, and belongs to the expected repository. Return structured metadata — PR title, merge date, author — that gets stored alongside the milestone in the indexer DB.

[Ugasutun]

@teethaking Can I work on this?