feat(rules): move a rule between global and project scope, keeping its id, history, areas and edges (#4063)

A rule's home is its reach: a rulebook topic makes it global, a project makes it
that project's. There was no way to change one, so a project rule decided to be
global could only be recreated and the original trashed — losing the id every
record cites, its edit history, its area tags and its relations.

- services.rulebooks.move_rule(rule_id, user_id, topic_id= | project_id=):
  exactly one destination (the model's CHECK), owned by the caller, not the
  rule's current home. A topic already holding a live rule with the same title
  is refused with a message naming that rule, instead of uq_rule_per_topic
  failing the commit. Someone else's rule reads as not found.
- Deliberately NOT done, and said in the docstring: no version (a version is
  what a rule said, milestone 323 decision 4), no duplicate gate (nothing new
  enters the corpus), no re-embed (retrieval reads the home at query time).
- Both doors: MCP move_rule, REST POST /api/rules/<id>/move (rule 33).
- UI: RuleHomePicker, one component in the rule editor (a global rule) and a
  project's rules tab (a project rule), so the two cannot drift on what a
  destination is.
- using-scribe names move_rule under "Where a new rule goes". Plugin
  2026.09.15.1626.

Milestone 414 step 3.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

docs/api-reference.md

@@ -115,6 +115,7 @@ endpoint at `/mcp`, not these REST routes.
 | GET | `/api/rules` | List rules |
 | POST | `/api/rulebook-topics/:tid/rules` | Add a rule to a topic |
 | GET / PATCH / DELETE | `/api/rules/:id` | Read / update / delete a rule |
+| POST | `/api/rules/:id/move` | Move a rule: `{topic_id}` makes it global, `{project_id}` makes it that project's |
 | GET | `/api/projects/:id/rules` | A project's own rules, and the global rules tagged to its areas |
 | POST | `/api/projects/:id/rules` | Create a project-scoped rule |