Add multi-repo registry onboarding

docs/registry-onboarding.md

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+# Registry Onboarding
+
+Multi-repo onboarding uses a repo-owned manifest to register ecosystem
+repositories with a running Railiance Fabric registry and to push graph and
+library inventory when the local checkout has the required inputs.
+
+## Run The Railiance Manifest
+
+Start the registry:
+
+```bash
+railiance-fabric-registry --db .railiance-fabric/registry.sqlite3 --port 8765
+```
+
+Sync the known local Railiance ecosystem repos:
+
+```bash
+railiance-fabric registry sync-manifest registry/railiance-repos.yaml
+```
+
+Use `--json` for automation and `--strict` when unavailable repos or invalid
+inputs should fail the command.
+
+## Manifest Shape
+
+```yaml
+apiVersion: railiance.fabric/v1alpha1
+kind: RegistryOnboardingManifest
+registry_url: http://127.0.0.1:8765
+repositories:
+  - slug: railiance-fabric
+    name: railiance-fabric
+    path: ..
+    default_branch: main
+    state_hub_repo_id: 2c0de614-e468-4eb6-8157-470649ac8c05
+    declaration_paths:
+      - ..
+    sbom: bom.json
+```
+
+`path` is the local checkout used for git metadata and default graph discovery.
+`declaration_paths` is optional; when omitted, the repo path is scanned for a
+`fabric/` directory. Relative paths are resolved from the manifest file.
+
+`sbom` or `sboms` may point to CycloneDX JSON/YAML files. When present, the
+command ingests them as queryable library inventory after repository
+registration.
+
+## Onboarding Behavior
+
+Each repository entry is registered first. If the checkout is unavailable or has
+no Fabric declarations yet, the command leaves the repo registered and reports a
+warning. This lets the registry represent known repos before every repo has
+adopted local declarations.
+
+When declarations exist, the command validates them, builds a graph snapshot,
+and posts it to:
+
+```text
+POST /repositories/{repo_slug}/snapshots
+```
+
+The first Railiance manifest keeps the seed ecosystem graph in
+`railiance-fabric` and registers adjacent repos as known sources. As those repos
+adopt repo-local `fabric/` declarations, they can be synced without changing the
+registry API.