Merge the open PRs

Snapshot the open PRs. That list is the work set. Merge each PR through the forge, one at a time, in an order that keeps later PRs updateable onto the default branch. Do not assemble independent PRs into a stack or replace the work set with one combined PR.

Standing rules:

• The work set does not grow. A PR opened after the snapshot is ignored. After every merge, compare remaining open PRs to the work set; never merge extras.
• A default-branch move is in scope. After each merge the default branch has moved. Update the next PR onto it, resolve, and re-verify before merging.
• Every PR lands through the forge. Never merge into a local default branch and push, and never push straight to the default branch. Do local git work only on the PRs' own branches, in a temporary worktree you remove afterwards.

1. Snapshot the work set