fix: update macOS quarantine guidance

[MrDoudou]

Summary

• replace the removed Homebrew --no-quarantine option with a supported post-install xattr command
• keep Control-click → Open as the preferred Gatekeeper workflow
• make the direct .dmg instructions use the same quarantine guidance
• prevent generated casks from reintroducing --no-quarantine

Validation

• bash -n scripts/gen-homebrew-cask.sh
• generated and validated both tap and --core cask variants for v0.30.0
• verified the tap variant matches the corrected published cask
• ruby -c on both generated variants
• brew style on the generated tap cask
• git diff --check

Fixes #176

[kipavy]

Thanks — this is the right fix. I retargeted it from main to dev, which is where feature and fix branches land here; main only moves on a release merge. Nothing else needed from you.

Keeping --core was the better call than collapsing the two variants: core is hostile to caveats that talk the user out of Gatekeeper, not only to --no-quarantine, so the xattr line belongs in the tap variant alone.

The rest of the issue's surfaces are covered alongside this:

• fix: drop the --no-quarantine hint from the cask and README homebrew-voltius#2 — the published cask caveat and tap README, so existing users stop seeing the broken hint before the next release regenerates the cask. Its cask is byte-identical to gen-homebrew-cask.sh v0.30.0 on this branch.
• fix(landing): drop the --no-quarantine Homebrew hint web#15 — landing page download card
• docs(install): drop the --no-quarantine Homebrew hint docs#17 — docs site install page
• docs(packaging): explain what keeps the core cask variant apart #178 — one stale paragraph in packaging/README.md that still describes the --no-quarantine hint; merge after this.