Use absolute issue links and link open-question issues

[Sirajmx]

What

• Fix 404 links. The README's Comment, Change request, Implementation feedback and Discussions links, and the two links at the end of docs/open-questions.md, were relative (../../issues/new…). GitHub resolves those under /blob/<branch>/, so they returned 404. They now use the full repository URL, matching the issue templates.
• Link the open-question tracking issues. docs/open-questions.md now links each question to its issue instead of Issue: TBD:
  • OQ-1 OQ-1 — Does per-field mutability belong in 3.0? #2 · OQ-2 OQ-2 — Do seller catalogs (catalog_ref) belong in 3.0? #3 · OQ-3 OQ-3 — Cross-channel guarantee comparability is unsolved #4 · OQ-4 OQ-4 — The format registry has no entries for audio/*, ctv/* or dooh/* #5 · OQ-5 OQ-5 — Is a min_line_items field needed? #6 · OQ-6 OQ-6 — Should payment_terms and invoicing_party be catalog references? #7 · OQ-7 OQ-7 — environments[] needs validation against real inventory #8
  • The maintainer note asking for these issues to be filed is replaced with a link to the open-question label.

Type

Editorial. No change to spec/.

Checks

Examples parse, Issue forms parse and Appendix A matches the body pass on the branch.

Please merge with Create a merge commit to keep the two commits separate.