feat(onboarding): redesign repo integration journey

custodian_cli.py:
- register-project now writes CLAUDE.custodian.md (suggestion) instead
  of overwriting CLAUDE.md; includes preamble with integration instructions
- registers repo via POST /repos/
- creates a "Repo Integration: {slug}" workstream in the domain's topic
  with 4 onboarding tasks (integrate CLAUDE.md, first workplan, SBOM,
  EPs/TDs); checks for existing workstream to be idempotent
- fixes {REPO_SLUG} template substitution (previously missing)

dashboard:
- repos.md: fetches workstreams; detects active repo-integration-* slugs;
  adds "Integrating" KPI card; shows ⚙ integrating badge per repo in
  coverage map and table; replaces "How to Ingest a Repo" with
  "Onboard a New Repo" 4-step panel with doc help button
- docs/repo-integration.md (new): full collaboration model doc — custodian
  as coach, repo agent as executor; journey, generated tasks, first session
  protocol, ongoing relationship
- docs/repos.md: links to new repo-integration doc; updates "What is a
  managed repo?" section; adds onboarding quick reference
- docs/reference.md: fix latent build error — code examples were in ```js
  fences (executed by OF); changed to ```javascript (display only)
- observablehq.config.js: adds "Repo Integration" to Reference nav

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

dashboard/observablehq.config.js

@@ -43,6 +43,7 @@ export default {
         { name: "Overview",                 path: "/docs/overview" },
         { name: "Progress Log",             path: "/docs/progress-log" },
         { name: "Reference & Context Help", path: "/docs/reference" },
+        { name: "Repo Integration",         path: "/docs/repo-integration" },
         { name: "Repos",                    path: "/docs/repos" },
         { name: "SBOM",                     path: "/docs/sbom" },
         { name: "Tasks",                    path: "/docs/tasks" },