* 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.
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