refactor(config): move embedder/l3 to top-level [embedder]/[l3] sections

They are shared infra (the orchestrator uses them for workflow repo-knowledge,
not just the router chat layer), so [router.embedder]/[router.l3] was misleading.
Loader/writer use the new names; legacy [router.*] still read as a fallback.
Also notes the STEERING LLM-reformulation cost as a ponytail follow-up.

core/router/src/main/kotlin/com/correx/core/router/RouterFacade.kt

@@ -186,6 +186,11 @@ class DefaultRouterFacade(
             emitIdeasCaptured(sessionId, ideas.ideas)
         }
 
+        // ponytail: STEERING launders the user's text through the router LLM (the `content` above)
+        // before injecting it as a note. For a clear instruction that's an extra inference that can
+        // distort intent; the reformulation only earns its cost when the input is terse/context-
+        // dependent. Upgrade path: inject the raw (validated) input directly and skip the rewrite
+        // unless a heuristic flags the message as too short/ambiguous to stand alone.
         val steeringEmitted = mode == ChatMode.STEERING && rawContent.isNotBlank()
         if (steeringEmitted) {
             val validationError = validateSteering?.invoke(content)