feat: expose snapshot restore planning API

[nicotsx]

The restore UI needs a target-scoped answer for the snapshot query root
and whether original-location restore is available. That decision now
belongs to core planning and must account for the selected restore agent
platform

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[nicotsx]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• ci: run restore integration checks #1025
• feat: use restore plans in snapshot restore UI #1024
• feat: expose snapshot restore planning API #1023 👈 (View in Graphite)
• feat: plan restores at execution target #1022
• refactor: add snapshot path context facade #1021
• fix: harden volume browser path codec #1020
• main

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