honor cross-repo Refs and skip unresolvable tasks in task_drift (#2112)
task_drift extracted only the issue number and looked it up in the PR's own repo, so a cross-repo Ref (owner/repo#N) resolved to the wrong repo, and an unresolvable/inaccessible task raised an uncaught GitHubAPIError that crashed the whole sweep. Use extract_tracking_ref to keep the owner/repo, look the task up in its actual repo, and skip (with a stderr warning) when it can't be resolved from this run (cross-org, private, deleted). close_drift then acts on the task's real repo. Surfaced by the scheduled sweep in logical-minds-foundry/.github choking on a PR that Ref'd vergil-project/vergil-tooling#2105 cross-org. Part of epic vergil-project/.github#80.