chore: address review feedback on registry-guideline automation

[sotashimozono]

Summary

Addresses review feedback on the registry-guideline automation PR that already merged into this repo (sibling of sotashimozono/Lattice2D.jl#23 / #35 on QAtlas). Three small fixes, all localised to .github/:

1. PRLabeler: the breaking label no longer sticks

Previously PRLabeler only ever added the breaking label based on the Type-of-Change checkbox. If a contributor checked the box, saved the PR body, and then unchecked it, the label remained — and since release-drafter uses breaking to force a minor bump, that stale label could silently promote a patch release into a minor one.

The workflow now toggles both directions: when the checkbox is unticked (or absent), it runs gh pr edit ... --remove-label breaking (error-tolerant, so it's a no-op when the label was never applied).

2. AutoRegister: unique $GITHUB_OUTPUT heredoc delimiter

The Generate release notes text step used a fixed RELEASE_NOTES_EOF delimiter when writing the wrapped body to $GITHUB_OUTPUT. The drafted release body is effectively user-controlled — a draft that happens to contain a line literally equal to RELEASE_NOTES_EOF would truncate the output and corrupt the @JuliaRegistrator register comment body.

Replaced with a random delimiter per run:

delim="RELEASE_NOTES_$(openssl rand -hex 16)"
{
  echo "content<<$delim"
  printf '%s\n' "$WRAPPED"
  echo "$delim"
} >> "$GITHUB_OUTPUT"

3. PR template: generic "breaking-changes section" wording

The checkbox description previously promised a `## Breaking changes` section in the drafted release notes. release-drafter's actual category titles vary across the fleet (💥 Breaking changes / ⚠️ Breaking Changes), so the exact header string was never guaranteed. Reworded to the generic "breaking-changes section" — same meaning, no false promise.

Test plan

• Tick and untick the Breaking change checkbox on a throwaway PR → label appears / disappears.
• Next time AutoRegister fires, the posted Registrator comment body is intact even for multi-line drafts.

Type of Change

• ✨ Feature (enhancement)
• 🐛 Bug Fix (bug)
• ⚡ Performance (performance)
• 📖 Documentation (documentation)
• 🧰 Maintenance (chore or refactor)
• 💥 Breaking change (breaking)

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