feat: add prismic slice upgrade-legacy

[angeloashmore]

Resolves: DT-3535

Description

Before this PR, only Slice Machine's /labs upgrader could upgrade legacy slices, the slices that the Legacy Builder defines inside a type.

After this PR, prismic slice upgrade-legacy upgrades one legacy slice at a time and says how its content changes. status, init, and the commands from #390 point at it. Content in Prismic does not change: the Content API returns upgraded slices through legacyPaths.

People and AI agents both run this command. When the right choice is unclear, the command lists the options and leaves the choice to the project's owner:

Slice "hero" already exists. How do you want to upgrade legacy slice "hero" of "landing_page"? This changes how editors pick slices, so the project's owner should choose:
  Add it to slice "hero" as a new variation: prismic slice upgrade-legacy hero --from landing_page --to hero
  Upgrade it to a separate slice: prismic slice upgrade-legacy hero --from landing_page --to <new-slice-id>

prismic init turning on the Type Builder is in #389.

Checklist

• If my changes require tests, I added them.
• If my changes affect backward compatibility, it has been discussed.
• If my changes require an update to the CONTRIBUTING.md guide, I updated it.

Preview

How to QA ¹

In a project with legacy slices, run npx prismic slice list --legacy, then npx prismic slice upgrade-legacy <id> --from <type>. The evals "upgrades a legacy slice" and "asks before combining legacy slices" cover the agent flow.

🤖 Generated with Claude Code

https://claude.ai/code/session_01H4jg7fmGVLhLJjo27eC7Mj

---

Note

Medium Risk
Changes custom type and slice models and editor-facing slice zones; mistakes in merge/variation or legacyPaths could mis-map live content until components are updated and pushed.

Overview
Adds prismic slice upgrade-legacy so legacy slices embedded in custom types can be converted to SharedSlice models locally, with legacyPaths wired for content compatibility after prismic push.

The command supports --from, optional --slice-zone, --to, and --variation (new slice, merge into an existing variation, or add as a variation). When a target slice already exists, it fails with guided options instead of picking a modeling choice. getLegacySlices now carries the slice zone for updating zone choices in place.

init, status, slice list, type view, and getSlice messaging now point at upgrade instead of “cannot edit legacy slices.” Agent evals cover single-type upgrades and asking before combining duplicate legacy IDs across types.

Coverage includes unit/integration tests and an isolated-repo push test.

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