feat(rules)!: a rule cannot be created, or edited into, having no trigger (#4099)

`when_to_apply` is not metadata. `rule_document` embeds a rule as
`{title} — {trigger}` / `When to apply: {trigger}\n\n{statement}`, the trigger
appearing twice so purpose dominates a short vector — the shape note 2485
measured on snippets (a 0.153 top-to-second gap against 0.010–0.023 for
everything else). Without one the document silently becomes title + statement:
a DIFFERENT shape, ranked against a corpus it does not match, with nothing to
report it. Every bar and every rank in the system assumes one shape.

`create_preference` has refused an empty trigger since it shipped. The two rule
creators defaulted it to "" — so the shape was enforced for the record kind
that guides and optional for the kind that binds.

The guard lives in the SERVICE, because both doors reach it: the MCP tools and
the frontend's fast path in routes/rulebooks.py. Written in either alone, the
other could still create a rule that never fires. The route keeps a matching
check for the STATUS CODE only (400, not the 404 it maps ValueError to).

update_rule refuses to EMPTY an existing trigger, checked after the mutation so
it covers `clear=[...]`, an emptied form input, and any route added later.
Deliberately asked as "did this edit remove one" rather than "does one exist":
a rule predating the guard has none, and refusing to save it would freeze
precisely the unreachable records that most need fixing.

Deliberately not following arose_from_id, which the human door exempts itself
from because provenance is about auditing what the AGENT changed. That reasoning
does not reach this field — a missing trigger is not a missing explanation, it
is a rule that does not work, and it fails an operator as badly as a session.

15 test fixtures across 6 files were creating rules with no trigger. They now
pass one; that they did not is the point — curation is not a guarantee.

Step 1 of milestone 416 "Retrieval stops guessing a bar". First because every
later step assumes one document shape, and it is much cheaper to guarantee
before a corpus grows than to backfill after.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

src/scribe/routes/rulebooks.py

@@ -345,6 +345,15 @@ async def create_project_rule(project_id: int):
     statement = (data.get("statement") or "").strip()
     if not statement:
         return jsonify({"error": "statement is required"}), 400
+    # Checked here as well as in the service, only for the STATUS CODE: the
+    # service raises ValueError, which this route maps to 404 for "project not
+    # found", and a missing trigger is a 400. The service stays the guard —
+    # this is the door telling the truth about whose mistake it was.
+    if not (data.get("when_to_apply") or "").strip():
+        return jsonify({
+            "error": "when_to_apply is required: a rule with no trigger never "
+                     "surfaces at the moment it applies."
+        }), 400
     title = (data.get("title") or "").strip() or statement.split(".")[0][:50]
     try:
         rule = await rulebooks_svc.create_project_rule(