Summarize changes in regeneration pull requests

[andrew]

Generate regeneration pull request descriptions from semantic changes to data/advisories.json. The description reports added, updated and removed advisories by source, the resulting index size, and a capped list of affected formulae.

Refresh the description when the standing regeneration branch is force-pushed so it stays in sync with the current diff.

For example:

Automated regeneration via `brew generate-vulns-advisories`, followed by rebuilding `data/advisories.json`.

- Added 12 generated advisories.
- Updated 10 generated advisories.
- The index now contains 56 advisories across 10 formulae.

Affected formulae (2):

- `glibc`
- `libssh2`

AI/LLM disclosure: I used Codex (GPT-5) to help implement this change. I reviewed the diff and exercised the generated description against a recent regeneration commit.

[Copilot]

Pull request overview

Adds automated, diff-based summaries for advisory regeneration pull requests, keeping the standing regeneration PR body synchronized with the current data/advisories.json semantic changes.

Changes:

• Introduces AdvisoryChangeSummary to render a Markdown summary (added/updated/removed by source, index counts, and capped affected formulae list).
• Adds rake advisories:summary and wires it into the regeneration workflow to generate a body file and use gh pr create/edit --body-file.
• Adds/updates specs to validate the workflow behavior and the summary rendering logic.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
spec/workflows_spec.rb	Updates workflow assertions and adds coverage for body refresh via gh pr edit --body-file.
spec/advisory_change_summary_spec.rb	New unit tests for summary rendering, formatting, caps, and duplicate-id detection.
Rakefile	Adds advisories:summary task that compares HEAD:data/advisories.json vs working tree.
lib/advisory_change_summary.rb	New renderer for regeneration PR summaries from advisory index JSON.
.github/workflows/regenerate.yml	Generates PR body file during regeneration and uses it for create/update PR body.

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