Preserve themes when updates fail

[27mfp]

Summary

• Stage theme files and generated templates before replacing the active theme.
• Preserve the active theme when copying, color generation, template rendering, or activation fails.
• Clone external themes into a staging directory so failed clones cannot delete an installed theme.
• If applying a cloned theme fails, restore the previous installed theme and retain the failed clone for inspection.
• Recover the previous active theme if activation is interrupted between the two atomic moves.
• Report post-activation integration failures as warnings after the new theme is safely active.
• Add hermetic tests for failure, interruption, warning, and success paths.

Validation

• bash test/omarchy-cli-test.sh
• bash -n bin/omarchy-theme-set bin/omarchy-theme-set-templates bin/omarchy-theme-install test/omarchy-cli-test.sh
• git diff --check

Release compatibility

This draft targets the current Omarchy 3 master implementation. Quattro is a separate major-release rewrite and will require a follow-up port if these safeguards are still desired there.

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