Add GO-2026-5617, GO-2026-5668, and GO-2026-5746 to .govulncheck-allow.
These are all daemon-side Docker/Moby vulnerabilities that don't affect
RedFlag because it uses the Docker client only for Ping, SecretList,
and container scanning — never for docker cp, archive operations, or
AuthZ paths.
Also update SECURITY.md to document all accepted exceptions in a table.
dep-scan.sh gates go/npm/cargo on push and bakes an attested posture into the
release — embedded in the server, signed into the manifest. Reasoning and the
two Moby exceptions are in SECURITY.md.
(posture-builder runs rustup; bookworm's cargo is too old for cargo-audit.)