docs(sbom): add SBOM reference page + withDocHelp on SBOM dashboard

- docs/sbom.md: what SBOM is, lockfile semantics, 5-level maturity standard,
  gap types A–E, per-ecosystem guidance, Syft OSS tooling, inter-repo task
  communication convention, ingest commands, compliance check commands
- sbom.md: wire withDocHelp(h1, "/docs/sbom") — ? button on page title
- observablehq.config.js: add SBOM entry to Reference nav section

EP-CUST-002 registered: Syft-based comprehensive SBOM generation
Task 5f8cade5 created: [repo:railiance-bootstrap] Add Ansible lockfile

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

state-hub/dashboard/observablehq.config.js

@@ -22,6 +22,7 @@ export default {
         { name: "Decisions", path: "/docs/decisions" },
         { name: "Decision Health", path: "/docs/decisions-kpi" },
         { name: "Progress Log", path: "/docs/progress-log" },
+        { name: "SBOM", path: "/docs/sbom" },
       ],
     },
   ],