feat(family): an idea links to the rule topic holding its norms - set_family_topic, the readouts name the rules, the idea row shows the link (milestone 463 step 1 gap, found in step 7, #4993)

Step 1 added family_ideas.topic_id but nothing could set it, so the note<->topic
link the milestone promised existed only as a column. set_family_topic links or
unlinks (0) a topic in a rulebook the caller owns, one idea per topic; the
version does not move. get_family_idea and get_family_adoption list the topic's
rules; list_family_ideas and the Family page name the topic. The model
docstrings no longer claim the topic's rules are platform-scoped in retrieval.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scribe/mcp/server.py

@@ -196,7 +196,7 @@ _WRITE_TOOLS = frozenset({
     "undo_family_decision",
     # family canon — the adoption ledger
     "revise_family_idea", "assess_family_adoption", "resolve_family_conflict",
-    "set_family_references",
+    "set_family_references", "set_family_topic",
     "bind_repo", "unbind_repo",
     # snippets, processes, the shape ledger
     "create_snippet", "update_snippet", "delete_snippet", "verify_snippet",

src/scribe/mcp/tools/family.py

@@ -372,12 +372,31 @@ async def set_family_references(note_id: int, snippet_ids: list[int]) -> dict:
     return {"references": refs}
 
 
+async def set_family_topic(note_id: int, topic_id: int) -> dict:
+    """Link a family idea to the rule topic that holds its binding norms — the
+    rules every project on its platforms follows. The note says when the idea
+    applies and why; the topic's rules say what must hold. One idea per topic.
+    get_family_idea and get_family_adoption then list those rules, so an
+    assessment judges against them.
+
+    Use it when an idea already has rules written for it, or when you write
+    rules that exist to carry one idea. The version does not move: linking
+    changes where the norms are found, not what they say.
+
+    Args:
+        note_id: the family idea.
+        topic_id: a topic in a rulebook you own (list_rulebooks,
+            list_topics). 0 unlinks.
+    """
+    return await family_svc.set_topic(current_user_id(), note_id, topic_id)
+
+
 def register(mcp) -> None:
     for fn in (
         list_family_ideas, get_family_idea, propose_family_idea,
         promote_family_idea, retire_family_idea, undo_family_decision,
         list_family_decisions, revise_family_idea, get_family_adoption,
         list_family_adoptions, assess_family_adoption, resolve_family_conflict,
-        set_family_references,
+        set_family_references, set_family_topic,
     ):
         mcp.tool(name=fn.__name__)(fn)