Stacked-PR loop

A stack is a chain of PRs where each depends on the one below it merging first (main <- A <- B <- C). Left unattended, a stack rots two ways: a lower PR updates and the ones above it go stale, or CI needs a nudge (a manual trigger comment, a required check that didn't fire) and nobody's watching. This is one check-in pass, meant to run on an interval, not a one-shot fix.

Step 1 — Map the current stack

List the PRs in the chain and their actual base branches (not assumed order — read each PR's base). Note which ones are merged, open, or closed-unmerged; a closed-unmerged PR downstream of it needs re-basing onto whatever the next real base is.

Done when you have the true current shape of the stack, bottom to top.

Step 2 — Rebase anything behind its base

For each open PR whose base has moved since the PR's branch was last updated (the base PR merged, or main advanced and this is the bottom of the stack): rebase the branch onto the current base and push. Do this bottom-up — rebasing a PR before the one below it is settled just means redoing the work.

Done when every open PR in the stack is rebased onto its actual current base, or you've confirmed it already was.

Step 3 — Pull real CI state, not a cached rollup

For each PR, get the live per-check state against its current head (a rebase invalidates prior runs). If a check is required but hasn't run, that's a red flag, not a pass — don't read absence as success.