chore: prepare public security checks

[jcwal1516]

Summary

• Add SECURITY.md and a pinned full-history Gitleaks workflow.
• Remove local sibling signinum patches so public clones resolve registry dependencies.
• Remove stale cargo-deny git source allowlist.

Test Plan

• cargo check --workspace
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• cargo package -p statumen --allow-dirty
• cargo deny check advisories bans licenses sources
• typos
• /tmp/gitleaks-8.30.1/gitleaks detect --source . --redact --verbose