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

tests/test_integration_rule_versions.py

@@ -57,6 +57,7 @@ async def constraint():
     book = await rulebooks_svc.create_rulebook(uid, "History fixtures")
     topic = await rulebooks_svc.create_topic(book.id, uid, "ci")
     rule = await rulebooks_svc.create_rule(
+        when_to_apply="when the moment this fixture stands in for arises",
         topic.id, uid, "The runner has no bash",
         "Write every `run:` step in POSIX sh.",
         why="the image ships no bash",
@@ -258,6 +259,7 @@ async def test_a_version_cannot_be_read_through_a_DIFFERENT_rule(constraint):
         rule = await s.get(Rule, constraint["rule_id"])
         topic_id = rule.topic_id
     sibling = await rulebooks_svc.create_rule(
+        when_to_apply="when the moment this fixture stands in for arises",
         topic_id, constraint["uid"], "A different rule", "Unrelated.",
     )