T8782: adopt central Mergify baseline via `extends: mergify`

Adds .github/mergify.yml that inherits from vyos/mergify:.mergify.yml.
The central baseline provides:
  - defaults.actions.backport.ignore_conflicts: false (new in T8782 —
    makes backport conflicts fail loudly instead of committing literal
    `<<<<<<< / ======= / >>>>>>>` markers; see incident below)
  - pull_request_rules → label conflicting PRs with `conflicts`
  - commands_restrictions → restrict @Mergifyio slash commands to
    @vyos/maintainers + vyosbot

Why this repo, why now: 2026-05-12, PR #1994 was backported via
`@Mergifyio backport sagitta circinus`. Cherry-pick conflicted in
docs/conf.py; Mergify's pre-T8782 default (`ignore_conflicts: true`)
committed the markers, the resulting PRs (#1998 / #1999) were merged
anyway, and Read the Docs builds on both branches broke with
`SyntaxError: invalid syntax (conf.py, line 309)`. Fix-forwards
#2000 (circinus) and #2001 (sagitta) restored both branches. Adopting
the central baseline closes the gap for this repo so the same failure
mode cannot recur via the Mergify-managed backport path.

Spec: https://vyos.atlassian.net/wiki/spaces/VYOS/pages/849477640
IS-432: https://vyos.atlassian.net/browse/IS-432

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