T8782: scaffold queue defaults and backport label vocabulary

[andamasov]

Summary

Follow-up to the 2026-05-13 Mergify kick-off meeting with Christian, Daniil, Kyrylo, Alex, and John. Codifies the next layer of the central baseline from the meeting outcomes without flipping any new live capability for the 2 current extenders (vyos/vyos-documentation and vyos/vyatta-cfg) or the ~27 T8852 migration targets.

Three additions, all in .mergify.yml:

• defaults.queue_rule: with update_method: rebase, merge_method: merge, batch_size: 1. Per the Mergify sharing docs, values under defaults: field-merge between parent and child — so any consumer that later defines a concrete queue_rules: entry inherits these and can override any single value. Inert until a consumer adds a queue_rules: block.
• Backport target label vocabulary as a comment block: backport/sagitta, backport/circinus, backport/equuleus, backport/crux. This is the canonical contract a future external GitHub Action will consume to implement Christian's "hold backport until target release tag exists" ask.
• Commented-out reference queue_rules: shape that the first opt-in consumer copies into its per-repo .github/mergify.yml. Validate on real PRs locally, then graduate the queue definition into this central file after one round of feedback.

Deliberately out of scope

• No queue_rules: entry centrally. Yuriy framed the meeting as "preparation, gather feedback, iterate" — declaring a concrete queue here would silently start accepting @Mergifyio queue on every consumer.
• No standalone update pull_request_rules. Alex's auto-rebase ask is covered by update_method: rebase once a queue is in use. A free-standing fleet-wide rebase action would force-push under maintainers' feet on every open PR.
• No auto-trigger backport rule. Christian's deferred-backport workflow needs an external trigger on tag creation (a GH Action listening on release.published); not a Mergify primitive.

Refs

• Meeting transcript: 2026-05-13 Mergify kick-off (Yuriy + Christian + Daniil + Kyrylo + Alex + John)
• Phorge master: T8782
• Fleet rollout subtask: T8852
• Change ticket: IS-432
• Confluence spec: https://vyos.atlassian.net/wiki/spaces/VYOS/pages/849477640
• Mergify inheritance semantics: https://docs.mergify.com/configuration/sharing
• Mergify queue rules: https://docs.mergify.com/merge-queue/rules

Notes for reviewer

• YAML parses (python3 -c "import yaml; yaml.safe_load(open('.mergify.yml'))"). Structural keys after this change: defaults (now with actions + queue_rule), pull_request_rules (unchanged, 2 entries), commands_restrictions (unchanged).
• The commands_restrictions.queue: *allowed line is unchanged. Because no queue_rules: entry exists, @Mergifyio queue from a maintainer would currently fail (no queue defined). No tightening needed at this layer.

Test plan

• CodeRabbit auto-review on flip-to-ready leaves no blocking findings (or all addressed/explained per rules)
• On merge: open a no-op PR on vyos/vyos-documentation (extender T8782: add central Mergify baseline for vyos org #1) and confirm Mergify continues to enforce only the existing rules (conflict label, T-ID format) and that @Mergifyio queue rejects with no-queue-defined
• On merge: same check on vyos/vyatta-cfg (extender general: T8595: add AGENTS.md #2)
• Open a follow-up issue/Phorge subtask for the first opt-in consumer to declare queue_rules: locally (per "graduate from per-repo to central after feedback")

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