mod: Ext 18 — ergonomic defrule / ruleset surface, 375/375

mod/defrule collects trailing conditions via &rest; mod/ruleset assembles rules.
No macro needed — conditions are plain data, fn supports &rest here. Produces
structurally identical rules to mk-rule (asserted) and works in the engine
unchanged. Closes the roadmap's original defrule surface. +11 tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/mod/defrule.sx

@@ -0,0 +1,16 @@
+;; lib/mod/defrule.sx — ergonomic rule / ruleset construction.
+;;
+;; The roadmap sketched a (defrule action :when conditions) surface. Conditions
+;; already evaluate to plain data, so this needs no macro — variadic functions
+;; suffice: mod/defrule collects its trailing condition forms via &rest (dropping
+;; the explicit outer (list ...)), and mod/ruleset assembles rules the same way.
+;;
+;;   (mod/ruleset
+;;     (mod/defrule "spam-hide" :hide (list :classification "spam"))
+;;     (mod/defrule "default-keep" :keep))
+
+(define
+  mod/defrule
+  (fn (name action &rest conds) (mod/mk-rule name action conds)))
+
+(define mod/ruleset (fn (&rest rules) rules))