document PR architectural incompatibility
- move design-pulls.md to sandbox/ (exploratory, not production-ready)
- add critical warning: PRs can't work via atproto records
- appview doesn't subscribe to sh.tangled.repo.pull in jetstream
- no firehose ingester for pulls (only issues have ingestIssue)
- web UI creates DB entries directly, atproto record is decorative
- improve release.md wording (current repo vs current release)
- add tree reminder to CLAUDE.md
PR support would require tangled-core changes (adding firehose consumer).
The design is architecturally sound but incompatible with current implementation.