## Why
Checking only the pull request head can miss conflicts with changes already on
`main`.
## What changed
Remove explicit pull request head refs from required workflow checkouts so
GitHub Actions checks out the synthetic merge commit by default. Keep the
blocking gate on the same combined revision as the child workflows it
evaluates, and document the merge-commit policy in the workflow README.
GitOrigin-RevId: dc357caa5dc43f46b9b9a0edea6ee560b4fe8efb
## Why
It's hard to change the set of required jobs when they're managed in the
GitHub UI, and when each workflow is responsible for choosing it's own
scheduling it's easy to end up with skew between what we enforce on PRs
vs. on main.
## What
- add a `blocking-ci` caller workflow, triggered by pull requests and
pushes to `main`, for Bazel, blob size, cargo-deny, Codespell,
`repo-checks`, rust CI, and SDK CI
- add an `always()` terminal job named `CI required` that fails unless
every called workflow succeeds
- add a `postmerge-ci` caller workflow for `rust-ci-full` and
`v8-canary`, with a terminal `Postmerge CI results` job
- centralize V8 relevance detection in `v8_canary_changes.py`; unrelated
PR and postmerge runs execute metadata only and skip the expensive build
matrices
- leave `v8-canary` outside the blocking gate and leave the external
`cla` check independent
## Rollout
A repository admin must replace the existing required GitHub Actions
contexts with `CI required` in the main-branch ruleset. Retain `cla` as
a separate required check. Until that change is coordinated, this PR
cannot satisfy the old standalone check names. In-flight PRs will need
to be rebased after this lands.
