v2.1-final

What's Changed

• [docs] MkDocs Material site, architecture diagrams, glossary, governance, threat model by @kpifinity in #48
• [chore] Code modernization: pyproject.toml, Pydantic v2, mypy strict, pre-commit by @kpifinity in #49
• [chore] Security & compliance hardening: cosign signing, SLSA L3, OSSF Scorecard, gitleaks, dependency review by @kpifinity in #50
• polish(pr4): contributor experience by @kpifinity in #51
• fix(ci): scope scorecard write permissions to the job by @kpifinity in #52
• fix(ci): pin trivy-action to v0.36.0 by commit SHA by @kpifinity in #53
• fix(lint): clean ruff and mypy on the deterministic core by @kpifinity in #54
• fix(security): restore bandit nosec annotation on dev health probe by @kpifinity in #55
• rfc(0002): SKI v3.0 — neuro-symbolic pivot (draft) by @kpifinity in #56
• fix(docs+ci): mkdocs build error on RFC 0002 + RUF100 in deployer.py by @kpifinity in #57
• docs(positioning): rewrite README, CITATION, docs/index for v3 framing by @kpifinity in #58
• spec(v3.0): draft normative specification by @kpifinity in #59
• feat(kg-validator): add v3 schema support behind --schema v3 by @kpifinity in #60
• feat(v3): add verdict envelope contract per spec v3.0 §4 by @kpifinity in #61

Full Changelog: v0.2.1...v2.1-final