TML-3221. The lint job's per-PR gates (`check:upgrade-coverage --mode
pr`, `check:release-notes --mode pr`) always diffed against
`origin/main`. For a stacked PR, which targets the branch below it, that
comparison sees the whole stack's diff in every PR. Each intermediate PR
then fails unless it repeats declarations its parent already carries —
so the stack-wide content had to ride the bottom commit, which breaks
self-containment: merging the stack partially would ship declarations
for changes that did not land.

## What changes

- The lint job now fetches and diffs against the branch the PR targets:
`github.base_ref`, falling back to `main` where `base_ref` is empty
(`merge_group`, `push`), which is also where main is the right
comparison.
- Both gates get `--prev "origin/$BASE"`. The scripts already accept
`--prev`; only the workflow hardcoded main.
- Comments on the checkout and fetch steps now describe the base-branch
behavior.

## Effect on stacked PRs

Each PR is judged on its own diff. A PR that changes `examples/`
declares its own upgrade instructions; a PR that does not, declares
nothing. Entries no longer need to pool in the bottom commit.

Window derivation is unchanged for the common case: a stacked base
carries the same root version as the head, so the coverage check derives
the same in-flight transition window it derived against main. If the
base is ahead of the head (the base took a release cut the PR has not
rebased onto), the check fails with an explicit "rebase" instruction
instead of a silent wrong-window comparison.

## Validation

- `pnpm lint:workflows` passes.
- Local mirror run on a real stacked pair from the raw-lane stack:
diffing the second PR against main passes only by inheriting the first
PR's declarations; diffing it against its actual base correctly demands
the second PR's own declaration. That demand is the fix working.

https://claude.ai/code/session_01NnNjsNcPMtbJZhnZz5Zzbe


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Chores**
* Improved automated validation so changes are evaluated against the
appropriate target branch.
* Aligned coverage and release-note checks to use the same relevant
baseline.
* This provides more accurate pull request feedback and helps prevent
misleading validation results when working from branches other than the
default branch.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: Oleksii Orlenko <robot@aqrln.net>