fix: add PR context to mirror linked-issue skip warnings

[polyjeff529]

## Summary

The eight `bt.logging.warning` calls in `_is_valid_linked_issue`
(`gittensor/validator/oss_contributions/mirror/scoring.py:445-488`)
log only the issue number when the issue is skipped from the
multiplier calculation. In a noisy multi-miner round dozens of
issues from unrelated PRs may all skip on the same condition (e.g.
`self-issue` or `closed too far from merge`), making it impossible
to trace a single skip back to its PR without grepping the
surrounding `_calculate_issue_multiplier` log line and reconstructing
the order.

## Fix

Same observability shape as the recently merged PR #711
("fix: add PR number context to file-content fetch failure
warning"): extend each warning with `PR #{pr_number} in
{repo_full_name}` context derived once at function entry as
`pr_ctx`. No behaviour change — the gate decisions and return values
are identical, only the log surface gains the context.

```python
pr_ctx = f'PR #{pr.pr_number} in {pr.repo_full_name}'

if li.is_transferred:
    bt.logging.warning(f'Skipping linked issue #{li.number} ({pr_ctx}) - transferred')
    return False
# ...same pattern for the other seven skip warnings