docs(release): add v0.7.0 release notes

[hanthor]

Refs #339

Adds docs/releases/v0.7.0.md, the release body for v0.7.0. It covers everything from v0.6.0 to main (135 non-merge commits). The v0.6.0 tag never got a GitHub Release, so the notes fold that line in and say so. That follows the recommended option in the #339 checklist.

Sections: VDI (#244, #260, #261), web dashboard (#294, #351, #364, #369, #370, #372, #373, #374, #384, #385), operations (#225, #251, #258, #278, #367), QEMU and bootc (#300, #333), move and export (#189, #314), plugins and supply chain (#253, #294, #301, #308, #315, #316, #323), and other changes (#356, #388, Renovate, tests). I checked each PR number: every one is merged after the v0.6.0 tag.

Still for a maintainer (not in this PR)

After merge, tag the release commit and publish it with this file as the notes:

git tag -s v0.7.0 <commit>
git push upstream v0.7.0
gh release create v0.7.0 --title "Corral v0.7.0" --notes-file docs/releases/v0.7.0.md

Then do the post-release checks in #339: confirm release-brew.yml updates the corral-vm formula, and re-check the #210 install-channel finding against the new tag. This PR says Refs, not Fixes, because those steps stay open.

Validation

• node ste-lint.mjs --max 0 (shared tuna-os/.github linter, .ste-budget is 0): 0 findings.
• Docs only, so no Go changes and no just ci run.

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