docs: improve model-router prose (cascade-lookup explanation, skill registration rationale)

org/system-model-router.org

@@ -113,8 +113,12 @@ Pluggable — set ~*complexity-classifier*~ to override.
 
 ** Cascade Lookup
 
-Finds the first ~(provider . model)~ entry in a cascade matching the
+The core iterates each backend in ~*provider-cascade*~ and calls the model
-current backend keyword. Case-insensitive.
+selector for each one. This function matches the current backend against the
+per-slot cascade list to find the appropriate model. Returns the first
+~:code~ ~(provider . model)~ entry whose provider matches, or ~nil~ if
+the backend has no entry in that slot's cascade (the core will skip to
+the next provider).
 
 ;; REPL-VERIFIED: 2026-05-03T14:00:00
 #+begin_src lisp
@@ -192,7 +196,11 @@ Reads cascade configuration from environment variables and registers
 
 ** Skill Registration
 
-Triggers on nothing (observer). Initialization happens at load time.
+The model router is an observer skill — it has no trigger and no
+deterministic gate. All work happens at load time via ~model-router-init~,
+which reads env vars and registers into the core ~*model-selector*~ hook.
+The ~defskill~ call exists only to register metadata (priority, name) for
+telemetry and lifecycle management.
 
 #+begin_src lisp
 (defskill :passepartout-model-router
@@ -205,3 +213,5 @@ Triggers on nothing (observer). Initialization happens at load time.
 #+begin_src lisp
 (model-router-init)
 #+end_src
+
+