Say what a stacked PR needs for CI to run

The stacked-PR guidance told you to verify that CI runs for every PR but
named nothing that makes it fail, so a stack that goes dark leaves you
guessing. Record the mechanism: `gh stack link` moves each base onto the
branch below, and both triggers in ci.yml filter on `master` and
`11.0_release`, which for `pull_request` matches the base.

Also record what is safe to do about it, since the obvious worry is
wasted CI: results attach to the head SHA and survive a later base
change, and re-linking mid-run cancels nothing, so there is no reason to
wait for CI before linking a stack.

Signed-off-by: Cristiano Calcagno <ccrisccris@gmail.com>

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W8g8qwBARAcvW9MyuKQq8H

Author: cristianoc

AGENTS.md

@@ -298,6 +298,20 @@ branches linear and in the same repository, and list branches or PRs from
 bottom to top. For existing PRs, use `gh stack link BOTTOM_PR [NEXT_PR...]`,
 then verify that GitHub reports stack metadata and runs CI for every PR.
 
+`gh stack link` rewrites each PR's base to the branch below it; only the bottom
+PR keeps `master`. That matters because CI keys off the base: both triggers in
+`.github/workflows/ci.yml` are filtered to `branches: [master, 11.0_release]`,
+and for `pull_request` that filter matches the *base* branch, with no `push`
+fallback for feature branches. So "verify CI runs for every PR" means watching
+a real push produce check runs on each one, not just reading the stack UI.
+
+If a PR in the stack goes dark, retarget it with `gh pr edit <n> --base master`,
+push, let CI finish, then re-link. Two properties make that safe, both observed:
+check runs attach to the head SHA, so completed results survive a later base
+change; and re-linking mid-run neither cancels in-flight runs nor starts new
+ones, because a base change is not one of the `pull_request` types the workflow
+listens for. There is no need to wait for CI before re-linking.
+
 ### Code Quality
 
 - Follow existing patterns in the codebase