* ci: run CI and the PR title check for the merge queue
A merge queue tests each queued change on top of the ones ahead of it and
waits for the required checks, CI result and PR title, on that merge
group. Both workflows now run on merge_group, reading the base and head
from either event. The title was checked on the pull request, so the
queue run reports success without a title to read.
* docs: explain stacked pull requests and the merge queue
Agents had to be told what a stacked pull request is each time.
CONTRIBUTING.md now covers stacks with gh stack, keeping them linear,
and landing them through the merge queue; AGENTS.md links to it, and
tools/AGENTS.md records that required checks must run on merge_group.
* docs: shorten the stacked pull request and merge queue guidance
The AGENTS.md line loads into every agent context, so keep it to the
rules; CONTRIBUTING.md keeps the commands.
CI and the PR title check only ran for pull requests targeting master,
so a pull request stacked on another one got no checks, and no CI
result gate, until the one below it merged. Both now run for every
base. Path classification already diffs against the pull request's own
base commit, so a stacked pull request is classified by its own changes.
