feat(ci): read queue-info metadata from the git note (CI-agnostic)

[sileht]

mergify ci queue-info read the merge-queue batch metadata from the
GitHub PR body — via the GHA event payload (no-arg) and a GitHub API
fetch by PR URL. Both are GitHub-specific and need a token.

The engine publishes the same TrainInfo YAML as a git note under
refs/notes/mergify/<mq_branch> on the MQ branch head commit, precisely
so CI providers can read it with plain git and no token. Make queue-info
read only that note for the current HEAD, dropping the PR-body and
PR-URL paths entirely. It now works in any CI (GitHub Actions, GitLab,
CircleCI, Jenkins, ...), takes no arguments and needs no token.

• Emit the note's full payload verbatim as JSON, parsed into a generic
  value rather than a fixed struct, so every engine field (top-level and
  per-PR scopes, and anything added later) shows up without a CLI
  change.
• Enumerate refs/notes/mergify/* and return the note attached to HEAD
  instead of deriving the branch name, so it works under a detached HEAD
  (how most non-GitHub CIs check out a revision) with no CI-specific env.
• Share the note read + git plumbing via mergify-ci/src/git.rs
  (read_note, succeeds, capture); git_refs::real_notes_reader
  reads the same note and pulls just checking_base_sha. Drop the
  now-unused queue_metadata::detect and the dead parse_notes_payload.

Fixes MRGFY-7664

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com

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[sileht]

Revision history

#	Type	Changes	Reason	Date
1	initial	8ef9de2		2026-06-16 07:28 UTC
2	content	8ef9de2 → 2694939 (raw)		2026-06-16 07:28 UTC
3	content	2694939 → f6db5e2 (raw)		2026-06-16 08:18 UTC
4	content	f6db5e2 → 3305031 (raw)		2026-06-16 12:27 UTC

Pull request has been modified.

[mergify]

Queued — the merge queue status continues in this comment ↓.

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-06-16 13:54 UTC · Rule: default
• ✅ Checks passed · on draft merge queue: queuing main (9e9cae5) and #1600 together #1611
• ✅ Merged — 2026-06-16 14:03 UTC · at 3305031914be5273f2c84ec43903b297297a7eaa · squash

This pull request spent 8 minutes 30 seconds in the queue, including 7 minutes 52 seconds running CI.

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