docs: define and test the supported migration path from bluefin to dakota

[castrojo]

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  next action:  same bug? ujust confirm 680

---## Problem
There is no documented or tested migration path for existing bluefin users to move to dakota. Specifically:

• Is bootc switch from bluefin to dakota supported? If so, what are the prerequisites?
• Is fresh install the only supported path? If so, what happens to user data?
• Is there a rollback path if the migration fails?
• What is the data preservation story (home directory, installed Flatpaks, dconf settings)?

projectbluefin/bluefin-lts:README.md explicitly documents that rebasing between families is unsupported and requires fresh installs. No equivalent documentation exists for bluefin → dakota.

This ambiguity is a release risk: users will attempt migration without a supported path.

Needed

1. Define the official migration policy: "fresh install only" OR "bootc switch supported with conditions"
2. If fresh install: document data backup/restore expectations
3. If bootc switch: test it in CI (at minimum in QEMU) and document prerequisites
4. Add migration docs to the user-facing README

Found during factory gap analysis (2026-06-03).