feat(rules): a preference updates without asking, and says what taught it (#3849 step 2)

The write path, and the step where a preference stops being a relabelled
rule. `create_preference` / `update_preference` on the MCP surface, plus
`kind` on update_rule and both HTTP doors.

SEPARATE TOOLS, NOT A `kind=` ARGUMENT. create_rule's docstring IS the
approval gate (#3557): propose, offer three answers, wait. That is right for
a rule — the person it binds should have agreed. A preference inverts it, and
one reached through create_rule would be read through that prose, so the
caller would hesitate over exactly the act this kind exists to make routine.
Two doors, two contracts, one table. Reads stay shared: a preference IS a
rule row, and "what governs this" wants both.

Two required fields, each buying something:

- `when_to_apply`, because the trigger is two-thirds of the embedded
  document. Without one the record is written, stored, and silently never
  delivered — indistinguishable from one nobody wrote.
- `arose_from_id`, the price of the ungated write. A corpus that drifts with
  no record of what taught each change cannot be audited, and the operator's
  veto over drift is worth exactly as much as their ability to read why it
  happened.

The near-duplicate gate is what lets this corpus be written freely and stay
small: the second preference about a thing updates the first. It is
title-scoped and kind-blind, so it also catches a preference restating a rule
that already binds.

The asymmetry is guarded as two PRESENCE facts — the rule door still asks,
the preference door still says write it — never as an absence. An absence
check passes against a docstring that was deleted or rewritten into something
else, which is snippet #3352's warning and would read as coverage here while
proving nothing.

`_plain_detail` moved to tests/helpers on its second copy, per that module's
own reason for existing (#2825).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011cPyzNnegXHr5iRMzzy5KJ

src/scribe/routes/rulebooks.py

@@ -178,6 +178,11 @@ async def create_rule(topic_id: int):
             order_index=data.get("order_index", 0),
             when_to_apply=data.get("when_to_apply", ""),
             tier=data.get("tier", "always_on"),
+            # The human door carries `kind` too, and without the MCP door's
+            # required provenance: an operator editing their own preference
+            # owes nobody an explanation. That requirement is about auditing
+            # what the AGENT changed, not what they did themselves.
+            kind=data.get("kind", "rule"),
             arose_from_id=data.get("arose_from_id", 0) or 0,
             verify_with=data.get("verify_with", ""),
             expires_when=data.get("expires_when", ""),
@@ -212,7 +217,7 @@ async def update_rule(rule_id: int):
     fields = {
         k: v for k, v in data.items()
         if k in ("title", "statement", "why", "how_to_apply", "order_index",
-                 "when_to_apply", "tier", "arose_from_id",
+                 "when_to_apply", "tier", "kind", "arose_from_id",
                  "verify_with", "expires_when")
     }
     # No clear_fields here: a form sends "" for an emptied input, and the
@@ -436,6 +441,11 @@ async def create_project_rule(project_id: int):
             order_index=data.get("order_index", 0),
             when_to_apply=data.get("when_to_apply", ""),
             tier=data.get("tier", "always_on"),
+            # The human door carries `kind` too, and without the MCP door's
+            # required provenance: an operator editing their own preference
+            # owes nobody an explanation. That requirement is about auditing
+            # what the AGENT changed, not what they did themselves.
+            kind=data.get("kind", "rule"),
             arose_from_id=data.get("arose_from_id", 0) or 0,
             verify_with=data.get("verify_with", ""),
             expires_when=data.get("expires_when", ""),