Implement path for deep, automated major upgrades

Clear and concise description of the problem

Stacks has an upgrade command which lets the user safely automate major version updates.

Suggested solution

buddy-bot could include these diffs in their PRs (if configured to do so)

and we could let libs/packages know (that want to be automatically supported for this) that they would need $pkgName upgrade --from $version -- to $version

Alternative

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guide.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

Resolved by #1388 (39c5abc).

src/upgrades/ implements the path this issue asked for: for a major update it finds where the package is actually used, analyses the version span's breaking changes against that usage, and either reports a migration plan or attempts the migration and verifies it against the repository's own tests.

The ladder is deliberate — with no AI configured, behaviour is exactly what it is today; with AI it adds a report; only with ai.majorUpgrades.autoMigrate does it change code, because a wrong migration costs far more than a missing one. Unverified migrations open as drafts and say so.

The remaining wiring (multi-release span notes, and calling this from createPullRequests) is tracked in #1396.