ci: scan security on maintained Go and preserve minimum compatibility

[strider2038]

Changes

Share required verification between PR CI and maintainer-dispatched releases: existing quality gates, minimum-Go build/tests, and Security using govulncheck v1.8.0 with Go 1.26.6. Security fails on findings and scanner/setup errors and retains summaries and scanner output artifacts. There is no legacy security job in CI or releases; old-toolchain security audits can be performed manually when needed.

Release documentation explains the compatibility/security policy and identifies quality/minimum/Security as the required checks to configure in branch protection. Hosted rules currently have no required checks configured.

Validation

• actionlint, shell syntax and git diff --check passed.
• Scanner fixtures verify clean results, findings, operational errors, missing scanner and failed toolchain preflight; every nonzero scanner result fails.
• Prior CI passed all library checks (Squirrel fuzz timed out once and passed on retry). CI reruns on this update.
• No release or tag was created.