feat(rules): preferences are writable, and their drift arrives (#3895)

Milestone 399 step 5. Steps 1-4 put preferences into the backend: a kind
column, an inverted write path, a third register in the injected block, a
delivery slot. Nothing the operator could touch. Rule 27 forbids leaving it
there, and here it matters more than usual, because the UI is the only
guard against the risk the milestone named up front — an agent misreads one
session, rewrites a preference, and follows the rewritten version forever
while the operator never sees the moment it changed.

Four things ship.

A preference is DISTINGUISHABLE. `kind` reaches the client (the server has
always sent it in rule_brief) and a preference carries a chip. Force is the
one thing a list of instructions must not leave the reader to infer, and a
row that renders identically to a rule teaches the opposite of both facts
about a preference: it does not bind, and a session may rewrite it.

A preference is WRITABLE. The editor gains the kind as a first-class choice
with the test beside it — what happens when someone does not do this — and
says plainly, when preference is chosen, that sessions rewrite these
without asking and every rewrite is kept.

DRIFT ARRIVES. `GET /api/rules/drift` returns one row per rewritten
preference carrying its latest rewrite: what it said, what it says now, and
the record named by `arose_from_id` that taught the change. Both texts ride
along so the list shows the diff without a call per row. The new pane sits
beside the staleness sweep, because drift belongs to no one rulebook, and
it answers a question the operator would not have thought to ask.

REVERSION IS ONE ACTION, and this is the carve-out worth arguing with.
Milestone 323 refused a one-click restore for rules — "a binding
instruction should not be revertible in one click", because a silent revert
erases the only record of why the rewrite happened. That reasoning turns on
the rewrite being the operator's own decision. A preference's is not: the
agent makes it mid-work without asking, so reverting is a veto over someone
else's edit rather than an undo of your own, and a veto costing more than a
shrug is not supervision. The route refuses anything but a preference (409),
and nothing is erased: the restore goes through update_rule, so it snapshots
too and the history GAINS the revert. An integration test pins that, because
it is the whole basis for the exception.

Tested against real Postgres — every claim is about which rows come back
and in what order, which a stand-in session cannot judge.

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

src/scribe/routes/rulebooks.py

@@ -258,11 +258,57 @@ async def get_rule_version(rule_id: int, version_id: int):
     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
+# NO restore route FOR A RULE, 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.
+#
+# A PREFERENCE IS THE EXCEPTION, and the route below refuses anything else.
+# 323's reasoning turns on the rewrite being the operator's own decision.
+# A preference's rewrite is not: the agent makes it mid-work without asking,
+# which is what the kind is for. Reverting one is a veto over someone else's
+# edit rather than an undo of your own, and milestone 399 named the cost of
+# making that veto expensive — drift supervised in name only. The safeguard
+# 323 actually wanted survives intact, because the restore goes through
+# update_rule too: the rewrite stays in the history with the revert recorded
+# after it, so the history gains an entry rather than losing one.
+
+
+@rulebooks_bp.get("/rules/drift")
+@login_required
+async def preference_drift():
+    """Preferences that have been rewritten, most recently changed first.
+
+    One row per preference carrying its latest rewrite, what it said before,
+    and the record that taught the change. `?limit=` caps the list.
+    """
+    uid = get_current_user_id()
+    try:
+        limit = int(request.args.get("limit", 20))
+    except (TypeError, ValueError):
+        limit = 20
+    rows = await rulebooks_svc.recent_preference_drift(uid, limit=limit)
+    return jsonify({"drift": rows})
+
+
+@rulebooks_bp.post("/rules/<int:rule_id>/versions/<int:version_id>/restore")
+@login_required
+async def restore_rule_version(rule_id: int, version_id: int):
+    """Put a preference back to what that version said. Preferences only.
+
+    409 rather than 400 on a rule: the request is well-formed and the caller
+    is not wrong to have asked — this rule is simply in a state where the
+    action does not apply, and the message says which state and why.
+    """
+    uid = get_current_user_id()
+    try:
+        rule = await rulebooks_svc.restore_rule_version(rule_id, version_id, uid)
+    except ValueError as exc:
+        return jsonify({"error": str(exc)}), 409
+    if rule is None:
+        return jsonify({"error": "rule or version not found"}), 404
+    return jsonify(await rulebooks_svc.rule_detail(uid, rule))
 
 
 @rulebooks_bp.post("/rules/<int:rule_id>/relations")