fix(installer): recover diverged managed updates

[LeonSGP43]

Summary

• detect when the managed install clone and origin/main have both advanced after fetch
• reset the managed clone to origin/$BRANCH instead of aborting on ff-only pull
• add a regression test that pins the divergence probe and reset fallback

Verification

• bash -n scripts/install.sh
• uv run --with pytest python -m pytest tests/test_install_sh_diverged_update.py
• git diff --check

Fixes #53257

[tonydwb]

Code Review Summary

Verdict: Approved

Clean fix for the installer to recover from diverged managed clones. When git pull --ff-only fails because both local and remote advanced, the installer now detects the divergence and resets to the remote branch. Good test coverage with dedicated regression tests.

Looks Good

• Correct divergence detection using rev-list --left-right --count
• Proper recovery with git reset --hard
• Well-tested with regression tests for issue #53257
• No security concerns

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Reviewed by Hermes Agent

[Chewbee]

I guess the fix was not publisehd yet (thanks for fixing that fast) as the behavior is exactly the same as we speak

[teknium1]

Thanks for the focused installer recovery fix. Current main now provides this guarantee through the later superseding work referenced by #55663.

• scripts/install.sh:1229-1232 retries a failed ff-only update by resetting the managed checkout to origin/$BRANCH.
• Commit a40f22798ec6b5877958f6dbe5aeda8468e7d628 implemented that behavior and also covered scripts/install.ps1.
• tests/test_install_diverged_update.py:44-67 verifies the fallback for both installer paths.
• The fix shipped in v2026.7.1.

Automated hermes-sweeper review.