Merged PRs can break develop even when each was CI-green at open

[kavin-114]

Two PRs can each pass CI when opened, because each was tested against an older develop, in isolation. Merging both can still break develop: neither was re-tested against the other's changes. This is the recurring "CI green at open is not correctness" class.

Expected: before a PR lands, it is re-tested against the current develop tip (plus anything ahead of it in line), and merges only if still green.

Proposal (do later): enable a Mergify merge queue. Mergify is already active on the repo (public, so free). Label-triggered opt-in, batches a few PRs per CI run, and bisects a failing batch so innocent PRs still land.

Sketch for .mergify.yml (adds to the existing backport rules):

queue_rules:
  - name: develop
    merge_conditions:
      - check-success=lint
      - check-success=frontend-build
      - check-success=frontend-tests
      - check-success=tests (1)
      - check-success=tests (2)
      - check-success=tests (3)
      - check-success=tests (4)
    merge_method: merge        # develop takes merge commits
    update_method: rebase      # rebase queue branch onto develop while testing
    batch_size: 3              # test up to 3 PRs in one CI run
    batch_max_wait_time: 5min
    checks_timeout: 45min

pull_request_rules:
  - name: queue approved + labeled develop PRs
    conditions:
      - base=develop
      - label=queue
      - "#approved-reviews-by>=1"
      - -conflict
      - -draft
    actions:
      queue:
        name: develop

Usage: approve a PR, add a queue label (or comment @mergifyio queue); Mergify rebases, re-tests, and merges when green.

Notes before enabling:

• If the develop ruleset has "require branches up to date before merging", turn it off; the queue guarantees up-to-date, and both on can deadlock.
• Validate the config with Mergify's config check / dashboard first (keys match Mergify's current schema; deprecated autoqueue / allow_inplace_checks intentionally omitted).
• Decisions open: queue label vs auto-queue every approved PR; batch_size 3 vs 1.

Full plan/discussion: Claude Code session https://claude.ai/code/session_01CbJcFctYibnXhwEQrpnHM3