fix(config): config writer silently dropped router.narration.model_id

Every TUI config-editor save regenerates the TOML through
CorrexConfigWriter, whose [router.narration] section never wrote
model_id — one save and narration silently fell back to the main
model, queueing every narration behind slow 9B inferences (live QA:
approval narrations arrived minutes late, after the workflow had
already failed). The writer now persists it and the field is editable
in the TUI (blank clears it).

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

@@ -80,6 +80,7 @@ object CorrexConfigWriter {
         b.kv("top_p", cfg.router.narration.topP)
         b.kv("max_tokens", cfg.router.narration.maxTokens)
         b.kv("max_per_run", cfg.router.narration.maxPerRun)
+        cfg.router.narration.modelId?.let { b.kv("model_id", str(it)) }
 
         b.section("orchestration")
         b.kv("stage_timeout_ms", cfg.orchestration.stageTimeoutMs)

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

@@ -135,6 +135,11 @@ object EditableConfig {
             getString = { it.router.narration.maxPerRun.toString() },
             withString = { c, v -> nar(c) { it.copy(maxPerRun = int("router.narration.max_per_run", v)) } },
         ),
+        EditableField(
+            "router.narration.model_id", ConfigFieldType.STRING,
+            getString = { it.router.narration.modelId.orEmpty() },
+            withString = { c, v -> nar(c) { it.copy(modelId = v.trim().ifBlank { null }) } },
+        ),
         EditableField(
             "personalization.enabled", ConfigFieldType.BOOL,
             getString = { it.personalization.enabled.toString() },