docs(security): create docs/security/ consumer verification documentation

[WilliamBerryiii]

OSSF Silver Criterion

documentation_security (SHOULD) — The project SHOULD document how consumers can verify the integrity and provenance of released artifacts.

Description

Create a docs/security/ directory with consumer-facing documentation covering attestation verification, SBOM inspection, tag signature verification, and a security overview index. This is the capstone documentation deliverable for the v2.8.0 supply chain security milestone — it makes attestation and signing capabilities discoverable and usable by downstream consumers.

Acceptance Criteria

• docs/security/README.md created with security overview and verification index
• docs/security/attestation-verification.md — how to verify build provenance with gh attestation verify
• docs/security/sbom-verification.md — how to inspect SPDX-JSON SBOMs generated by anchore/sbom-action
• docs/security/tag-signature-verification.md — how to verify gitsign-signed release tags
• docs/security/container-verification.md — how to verify container image attestations
• SECURITY.md enhanced with "What Gets Signed" attestation summary (reference: hve-core pattern)
• All verification commands use concrete examples from edge-ai artifacts

Cross-Repo Alignment

• hve-core: Has docs/security/ with attestation verification guide — use as reference template
• physical-ai-toolchain: Consumer verification documented inline in release notes — this issue formalizes that pattern

Dependencies

• Depends on: ci(security): establish security review gate via CODEOWNERS, PR template, and label #171 (security review gate establishes docs/security/ scope)
• Depends on: [FEATURE] Replace SLSA attestation with enterprise-compliant actions/attest #100 (attestation patterns must be implemented before documenting verification)
• Depends on: ci(release): implement Sigstore keyless signing for release tags #172 (tag signing must be implemented before documenting tag verification)
• Depends on: ci(containers): add GitHub-native attestation for container images #173 (container attestation must be implemented before documenting container verification)