feat(RMGR-WP-0008): add workplan and register receiving surfaces

docs/repository-registers_v0.1.md

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+# Repository registers v0.1
+
+Repository registers are ordinary, versioned YAML files under `registers/`.
+They are authoritative in the repository; Repo Manager's index is a replaceable
+projection.
+
+## Shared envelope
+
+Each register uses one file named `registers/<kind>.yaml`:
+
+```yaml
+schema: repo-manager.register.v0
+kind: technical-debt
+entries:
+  - id: TD-001
+    title: Coupled persistence
+    status: open
+    severity: high
+    created: "2026-08-21T10:00:00+00:00"
+    updated: "2026-08-21T10:00:00+00:00"
+    notes: []
+```
+
+Entry ids are stable within their register. `created` is immutable; mutations
+advance `updated`. Deletion-compatible routes map to `defer`, preserving the
+entry and Git history. Notes are append-only objects with `content`, `author`,
+and `created`.
+
+## Kinds and compatibility fields
+
+| Kind | Required create data | State Hub compatibility |
+| --- | --- | --- |
+| `sbom-inventory` | `package_name`, `ecosystem` | package/version/licence entries; snapshots and reports are derived |
+| `repo-goals` | `description` | repository goal records |
+| `upstream-contributions` | `type` | contribution type and target metadata |
+| `technical-debt` | none beyond id/title | severity, debt type, location, notes |
+| `extension-points` | none beyond id/title | priority, extension type, location |
+| `register-entries` | `register_kind` | generic extension register entries |
+
+Additional kind-specific fields are retained as YAML scalars, lists, or
+mappings. The envelope fields `id`, `title`, `status`, `notes`, `created`, and
+`updated` may only be changed through their governed command parameters.
+
+## Commands
+
+```bash
+rmgr register put --kind technical-debt --entry-id TD-001 \
+  --title "Coupled persistence" --data-json '{"severity":"high"}'
+rmgr register list --kind technical-debt
+rmgr register note --kind technical-debt --entry-id TD-001 --note "Owner assigned."
+rmgr register defer --kind technical-debt --entry-id TD-001
+```
+
+Mutations use the same expected-HEAD, idempotency, Git commit, optional
+push-seal, normalized-event, reindex, and dual-run-meter mechanism as workplan
+and task mutations.