Main's thread↔PR linking never reached the v2 runtime: the client types
were optional stubs and the v2 server dropped the field, so linking a PR
on a v2 environment silently no-opped and #7454's settle-once logic could
never match the linked identity.

The link now flows end to end: thread.metadata.update carries an optional
linkedPullRequest (object to link, null to unlink), the orchestrator
folds it into thread state, and both shell builders project it — no
migration needed since v2 shells persist as payload JSON. The client
command sends the field and the shell mapper surfaces it, so the existing
web/mobile badge and settle plumbing light up on v2 threads.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>