docs: v0.7 user-facing docs (INSTALL-V07, STRATEGY pivot, README, CHANGELOG)
A4 inline (agent failed on API):
- docs/INSTALL-V07.md: 130-line user walkthrough — bootstrap ISO,
Anaconda LUKS prompts, ostreecontainer pull, first-login TUI, day-
to-day bootc-upgrade / rpm-ostree-install / bootc-rollback.
- docs/STRATEGY.md: append PIVOT EXECUTION 2026-05-06 section
recording v0.5 ship, v0.6 cancel, v0.7 active.
- README.md: rewrite Quick install block for v0.7 path; legacy v0.5.0
block kept below.
- CHANGELOG.md: Unreleased entry covering the spike's CI port +
atomic CLI port + docs.