feat(rules): the history is readable — service, REST and MCP (#3242, milestone 323 step 3)

`list_rule_versions` / `get_rule_version` in the rulebooks service, a pair of
REST routes beside the note-version ones, and an MCP `rule_history` tool.

The ACL-scoped reads live in services/rulebooks.py rather than in
services/rule_versions.py because rulebooks already imports rule_versions for
the write path and the reverse would be a cycle. It is also the honest split:
rule_versions owns what a version IS, rulebooks owns who may read one.

Scoping is through the RULE, never the version's user_id, and both directions
of that mistake are now pinned by tests. That column is the ACTOR — scoping
by it would hand someone the snapshots they personally wrote on a rule that
has since moved out of their reach, and would hide from the rule's owner
every edit anyone else made. `get_rule_version` takes the rule id as well as
the version id so the ownership check and the fetch agree about which rule is
in play; the test for that uses a second rule the caller genuinely owns,
because a nonexistent id would pass on the ownership check alone and prove
nothing.

An unreadable rule returns None, not an empty list. The two mean different
things — "not your rule" versus "never reworded" — and the MCP tool keeps
them apart: None raises, empty says so in band.

THE DIFF QUESTION, ANSWERED — and the task's premise was half wrong. It says
"notes have DiffView.vue and a diff endpoint already". The component exists
and is reusable as-is: it takes `DiffLine[]` and nothing note-shaped, so step
4 can render a rule diff with it unchanged. The ENDPOINT does not exist —
diffs are computed client-side by `computeDiff` in useAssist.ts. So no diff
route is needed here, and none was written.

For the MCP door the answer is different again: an agent has no client to
compute a diff, but it also does not need one. Each entry holds the text the
edit REPLACED, so "what did this say before the most recent change?" is the
first entry, and the text that change produced is the rule as it stands. The
docstring says so, and a test pins that sentence — read the other way round,
every diff comes out backwards.

No restore, per the task. Putting an old wording back goes through
update_rule, which snapshots what it replaces, so the undo stays visible like
any other edit.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/scribe/routes/rulebooks.py

@@ -206,6 +206,42 @@ async def update_rule(rule_id: int):
     return jsonify(await rulebooks_svc.rule_detail(uid, rule, data.get("system_ids")))
 
 
+@rulebooks_bp.get("/rules/<int:rule_id>/versions")
+@login_required
+async def list_rule_versions(rule_id: int):
+    """A rule's edit history, newest first.
+
+    Listing form only — a rule's `statement` and `why` run to thousands of
+    characters, so a history list carrying every field would be unreadable
+    and expensive to send. Open one for the text.
+    """
+    uid = get_current_user_id()
+    versions = await rulebooks_svc.list_rule_versions(rule_id, uid)
+    if versions is None:
+        return jsonify({"error": "rule not found"}), 404
+    return jsonify({
+        "versions": [v.to_dict(include_text=False) for v in versions],
+    })
+
+
+@rulebooks_bp.get("/rules/<int:rule_id>/versions/<int:version_id>")
+@login_required
+async def get_rule_version(rule_id: int, version_id: int):
+    """One snapshot in full — what the rule said before that edit."""
+    uid = get_current_user_id()
+    version = await rulebooks_svc.get_rule_version(rule_id, version_id, uid)
+    if version is None:
+        return jsonify({"error": "version not found"}), 404
+    return jsonify(version.to_dict(include_text=True))
+
+
+# NO restore route, deliberately (milestone 323). A note version can be
+# restored; a binding instruction should not be revertible in one click.
+# Putting a rewrite back goes through update_rule, which takes its own
+# snapshot and leaves the undo in the history like any other edit — a silent
+# revert would erase the only record of why the rewrite happened.
+
+
 @rulebooks_bp.post("/rules/<int:rule_id>/relations")
 @login_required
 async def relate_rules(rule_id: int):