[dipto0321]

Summary

Cuts the v1.2.4 patch release, carrying the stale upgrade-marker fix merged in #142.

• The npm wrapper's version and binaryVersion both move to 1.2.4. binaryVersion is the tag the postinstall script downloads from, so it must match the release tag exactly — a mismatch would 404 every install.
• The CHANGELOG's [Unreleased] section becomes [1.2.4], with the compare links updated.
• Version references refreshed in README.md, CLAUDE.md, docs/managers.md, and SPEC.md.
• docs/release-checklist.md's wrapper-verification steps now take the version as a shell variable, so the next release doesn't need to edit them.

What ships in this release

The user-facing fix from #142:

• The "Detected an interrupted upgrade" warning now validates its own advice. If the snapshot it points at is still on disk, it's a genuine interrupted upgrade and the warning plus resume hint are unchanged (the bug(packages): restore aborts on first failed install; migration report never written #46 contract). If the snapshot is gone, the marker is stale — nodeup says so and clears it instead of printing a hint that cannot work.
• Clearing is race-safe: the marker is claimed with an atomic rename before it's inspected, so a concurrent nodeup upgrade that arms a fresh marker can't have it deleted out from under it.
• nodeup packages cleanup --stale-sentinel clears a stale marker on demand, and refuses to clear one whose snapshot still exists.

Test plan

• make ci green locally (0 lint issues; all packages pass under -race)
• go build ./..., go vet ./..., gofmt -l . clean
• Verified against the reported state: a stale marker is cleared with an explanatory message, a live marker keeps its warning and its resume hint
• GitHub Release carries 5 binaries + checksums.txt
• Homebrew cask at 1.2.4 with sha256s matching the release's checksums.txt
• Scoop manifest at 1.2.4 with the matching Windows hash
• npm view nodeupx version → 1.2.4, and a real npm install nodeupx@1.2.4 yields an unquarantined binary that runs
• Direct-binary download: sha256 matches checksums.txt, no quarantine, runs
• The installed v1.2.4 binary clears a stale marker and keeps a live one

Closes #141

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[cocogitto-bot]

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