Shield release reconciliation: preserve ancestry and avoid premature Linear completion

Observed 2026-09-04 UTC in e3-solutions/theft (GitHub PRs #791, #792, #793; Linear COR-4028): when a direct main→staging PR conflicts, resolve on a short-lived branch created from staging by merging main with a regular two-parent merge, regenerating any derived snapshot from the merged canonical sources, and PRing that branch back to staging. After it is green and merged, verify main is an ancestor of staging before opening staging→main. Also, linking the release ticket key to the prerequisite sync PR caused PR-merge automation to mark the whole release ticket Done prematurely; release workflows should prevent prerequisite PR completion from closing a multi-PR ticket.