feat(dashboard): Repos page with coverage map; expose last_sbom_at on RepoRead

- RepoRead schema: add last_sbom_at + sbom_source fields (already in model,
  now surfaced in API response)
- repos.md: new dashboard page — KPI row (total/domains/ingested/gaps),
  domain-grouped coverage map with SBOM/EP/TD chips, per-repo table with
  gap highlighting, domain filter + gap-only toggle, ingest how-to section
- observablehq.config.js: add Repos after Domains in nav

Coverage state: 3 repos registered (custodian×1, railiance×2);
2 ingested (the-custodian + railiance-hosts), 1 gap (railiance-bootstrap
— infra-only, no lockfile, expected)

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

dashboard/observablehq.config.js

@@ -8,6 +8,7 @@ export default {
     { name: "Decisions", path: "/decisions" },
     { name: "Progress", path: "/progress" },
     { name: "Domains", path: "/domains" },
+    { name: "Repos", path: "/repos" },
     { name: "Extension Points", path: "/extensions" },
     { name: "Technical Debt", path: "/techdept" },
     { name: "Contributions", path: "/contributions" },