Complete workplan state model cleanup

docs/workplan-convention.md

@@ -14,7 +14,7 @@ type: workplan
 title: "Short Title"
 domain: custodian
 repo: state-hub
-status: active
+status: proposed
 owner: custodian
 topic_slug: custodian
 ```
@@ -22,3 +22,14 @@ topic_slug: custodian
 During extraction, legacy `CUST-WP-*` plans may be bridged or migrated with
 their existing `state_hub_workstream_id` values. Write files first, then run
 State Hub consistency sync after this repo is registered.
+
+Canonical workplan/workstream statuses are:
+
+```text
+proposed, ready, active, blocked, backlog, finished, archived
+```
+
+Use `proposed` for a new plan that still needs review, `ready` after it has
+been checked against the current repo state, and `finished` when implementation
+is complete. `stalled` and `needs_review` are derived health labels, not stored
+frontmatter statuses.