chore: configure v6 maintenance releases

[martinrrm]

Summary

• Set templateOSS.backport to 6 so stable v6 releases use latest-6 rather than replacing the current latest dist-tag.
• Regenerate maintenance workflows using the existing pinned template-oss 4.30.0: pass the backport flag to both release helpers, default manual release CI to release/v6, and update the Dependabot directory fallback.
• Remove the maintenance branch copy of .github/settings.yml, as generated in backport mode; repository-wide settings remain owned by main.
• Add regression tests for stable/prerelease tag selection, generated helper arguments, and propagation of the computed tag to npm publish.

Package version 6.0.2, dependency ranges, Node engine support, CI matrices, and the existing Windows CI policy are unchanged. This PR does not publish a package or move registry tags.

Validation

Validated locally on Linux with Node 24.15.0 using the npm CLI checkout:

• Full package TAP suite: 16/16 test files passed, 100% statements/branches/functions/lines coverage.
• Owned ESLint passed.
• template-oss-check passed.
• git diff --check passed.

The multi-platform/Node CI matrix remains for GitHub Actions; it has not been run locally.