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

src/scribe/services/rulebooks.py

@@ -688,6 +688,71 @@ async def update_rule(
         return rule
 
 
+async def move_rule(
+    rule_id: int, user_id: int, *, topic_id: int = 0, project_id: int = 0,
+) -> Optional[Rule]:
+    """Give a rule a new home — into a rulebook topic (global) or onto a
+    project — keeping its id, history, Systems and relations (milestone 414).
+
+    A rule's home IS its reach: in a topic it applies to every project, on a
+    project to that project alone. Recreating the rule in the other home and
+    trashing the original would lose its id (and every record citing it), its
+    edit history, its area tags and its typed edges, which is why this exists.
+
+    Exactly one of `topic_id` / `project_id`, matching the model's CHECK
+    (migration 0059). Raises ValueError for: neither or both named, a target
+    the caller does not own, the rule already living there, or a topic that
+    already holds a live rule with this title (uq_rule_per_topic) — the message
+    names that rule, rather than letting the constraint fail the commit.
+    Returns None when the rule itself is not the caller's.
+
+    WHAT A MOVE DOES NOT DO, deliberately:
+
+    - No version. A rule's history records its TEXT (milestone 323, decision
+      4); its place is not text, and folding it in would make "version" mean
+      two things. The rule's `updated_at` moves; say why a rule moved where
+      the decision is recorded.
+    - No duplicate gate. Nothing new enters the corpus — the same rule changes
+      home — so there is no second record to warn about.
+    - No re-embed. The rule's document is its title, statement and trigger;
+      retrieval reads the home from the row at query time.
+    """
+    if bool(topic_id) == bool(project_id):
+        raise ValueError("name exactly one destination: topic_id (global) or project_id")
+    async with async_session() as session:
+        rule = await _fetch_owned_rule(session, rule_id, user_id)
+        if rule is None:
+            return None
+        if topic_id:
+            if rule.topic_id == topic_id:
+                raise ValueError(f"rule {rule_id} is already in topic {topic_id}")
+            await _assert_topic_owned(session, topic_id, user_id)
+            clash = (await session.execute(
+                select(Rule.id).where(
+                    Rule.topic_id == topic_id,
+                    Rule.title == rule.title,
+                    Rule.deleted_at.is_(None),
+                    Rule.id != rule.id,
+                )
+            )).scalar_one_or_none()
+            if clash is not None:
+                raise ValueError(
+                    f'topic {topic_id} already has a rule titled "{rule.title}" '
+                    f"(rule {clash}) — rename one before moving"
+                )
+            rule.project_id = None
+            rule.topic_id = topic_id
+        else:
+            if rule.project_id == project_id:
+                raise ValueError(f"rule {rule_id} is already on project {project_id}")
+            await _assert_project_owned(session, project_id, user_id)
+            rule.topic_id = None
+            rule.project_id = project_id
+        await session.commit()
+        await session.refresh(rule)
+        return rule
+
+
 # ── Edit history (milestone 323) ───────────────────────────────────────
 #
 # The ACL-scoped reads live HERE rather than in services/rule_versions.py,