docs: name the fork remote origin, as the docs already assume

[blaipr]

Summary

CONTRIBUTING.md tells contributors to git pull origin main and push branches to origin, while CLAUDE.md and docs/github.md described origin as the read-only nuxsmin/sysPass upstream and opus as the push remote. Following one document then the other pushed to the wrong place. This settles on the usual fork layout — origin is this fork, upstream is the read-only original.

Changes

• CLAUDE.md: pull/push examples use origin; the Remotes note becomes origin = OpusProjects/sysPass (push here), upstream = nuxsmin/sysPass (read-only)
• docs/github.md: same for the contribution workflow and the release-tag push, and its remotes sentence
• docs/github.md: its commit example still read Short title — what and why, contradicting the <type>: short imperative summary rule stated two paragraphs below it (docs: adopt the org's <type>: summary commit convention #816) — now consistent

CONTRIBUTING.md needed no change; it already assumed this layout.

Note

A clone that still names the remotes the old way needs git remote rename origin upstream && git remote rename opus origin to match.