fix: only show model switch success when persist succeeds (fixes #1435)

Previously, the /model command would display 'Model set to X' even when
the session state wasn't actually persisted (when sessionEntry, sessionStore,
or sessionKey were missing). This caused confusion as users saw success
messages but the model didn't actually change.

This fix:
- Tracks whether the model override was actually persisted
- Only shows success message when persist happened
- Shows a clear error message when persist fails

AI-assisted: Claude Opus 4.5 via Clawdbot
Testing: lightly tested (code review, no runtime test)

src/auto-reply/reply/directive-handling.impl.ts

@@ -288,6 +288,7 @@ export async function handleDirectiveOnly(params: {
     nextThinkLevel === "xhigh" &&
     !supportsXHighThinking(resolvedProvider, resolvedModel);
 
+  let didPersistModel = false;
   if (sessionEntry && sessionStore && sessionKey) {
     const prevElevatedLevel =
       currentElevatedLevel ??
@@ -346,6 +347,7 @@ export async function handleDirectiveOnly(params: {
         selection: modelSelection,
         profileOverride,
       });
+      didPersistModel = true;
     }
     if (directives.hasQueueDirective && directives.queueReset) {
       delete sessionEntry.queueMode;
@@ -447,7 +449,7 @@ export async function handleDirectiveOnly(params: {
       `Thinking level set to high (xhigh not supported for ${resolvedProvider}/${resolvedModel}).`,
     );
   }
-  if (modelSelection) {
+  if (modelSelection && didPersistModel) {
     const label = `${modelSelection.provider}/${modelSelection.model}`;
     const labelWithAlias = modelSelection.alias ? `${modelSelection.alias} (${label})` : label;
     parts.push(
@@ -458,6 +460,8 @@ export async function handleDirectiveOnly(params: {
     if (profileOverride) {
       parts.push(`Auth profile set to ${profileOverride}.`);
     }
+  } else if (modelSelection && !didPersistModel) {
+    parts.push(`Model switch to ${modelSelection.provider}/${modelSelection.model} failed (session state unavailable).`);
   }
   if (directives.hasQueueDirective && directives.queueMode) {
     parts.push(formatDirectiveAck(`Queue mode set to ${directives.queueMode}.`));