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/config/src/main/kotlin/com/correx/core/config/ConfigLoader.kt

@@ -419,8 +419,11 @@ object ConfigLoader {
         val toolsFileWriteSection = sections["tools.file_write"] ?: emptyMap()
         val toolsFileEditSection = sections["tools.file_edit"] ?: emptyMap()
         val routerSection = sections["router"] ?: emptyMap()
-        val routerEmbedderSection = sections["router.embedder"] ?: emptyMap()
-        val routerL3Section = sections["router.l3"] ?: emptyMap()
+        // Embedder + L3 are shared infra (the orchestrator uses them for workflow repo-knowledge,
+        // not just the router chat layer). Prefer the top-level [embedder]/[l3] sections; fall back
+        // to the legacy [router.embedder]/[router.l3] so existing configs keep working.
+        val routerEmbedderSection = sections["embedder"] ?: sections["router.embedder"] ?: emptyMap()
+        val routerL3Section = sections["l3"] ?: sections["router.l3"] ?: emptyMap()
         val routerGenerationSection = sections["router.generation"] ?: emptyMap()
         val routerNarrationSection = sections["router.narration"] ?: emptyMap()
         val modelsSection = sections["models"] ?: emptyMap()

core/config/src/main/kotlin/com/correx/core/config/CorrexConfigWriter.kt

@@ -56,13 +56,13 @@ object CorrexConfigWriter {
         b.kv("retrieval_k", cfg.router.retrievalK)
         b.kv("token_budget", cfg.router.tokenBudget)
 
-        b.section("router.embedder")
+        b.section("embedder")
         b.kv("backend", str(cfg.router.embedder.backend))
         b.kv("dimension", cfg.router.embedder.dimension)
         cfg.router.embedder.url?.let { b.kv("url", str(it)) }
         cfg.router.embedder.modelId?.let { b.kv("model_id", str(it)) }
 
-        b.section("router.l3")
+        b.section("l3")
         b.kv("backend", str(cfg.router.l3.backend))
         cfg.router.l3.persistPath?.let { b.kv("persist_path", str(it)) }
         b.kv("python_executable", str(cfg.router.l3.pythonExecutable))