swiftformat version sync

[AlexAxthelm]

Problem

swiftformat is installed with brew install swiftformat (latest) in CI, but
nothing is pinned locally. When Homebrew's stable moves ahead, CI's swiftformat
outruns local installs and the Format Check / swiftformat job fails on code
that make swift-format-check passes locally — an invisible-locally failure.

This just happened: the CI runner had 0.63.0 (released 2026-08-30) while a
local machine had 0.62.1. In 0.63.0, redundantReturn newly strips return
from single-statement closures with an explicit return type, so it flagged
iOS/Pollux/DatabaseManager.swift (untouched by the PR) while local checks were
clean.

Goal

Pin swiftformat to a single known version shared by CI and local, so formatting
is deterministic and version bumps are deliberate (a reviewable PR, not a silent
runner-image update).

Proposed

• Pin CI to a specific swiftformat version (currently recent: 0.63.0) instead
  of brew-latest — install the pinned release binary directly or via a
  version-pinning step, rather than brew install swiftformat.
• Record the pinned version where local installs can match it (a note in
  docs/HACKING.md and/or a .swiftformat-version file), plus the bump steps.
• Do the same for swiftlint (also brew-latest) to avoid the same class of drift.

Acceptance criteria

• CI uses a pinned swiftformat version; bumping it is an explicit PR.
• make swift-format-check locally and the CI job agree for the pinned version.
• HACKING/docs state the version and how to install/upgrade it.

Context

• swiftformat 0.63.0: https://github.com/nicklockwood/SwiftFormat/releases/tag/0.63.0
• The DatabaseManager redundant-return fix that unblocks the current PR is
  separate from this issue.