fix: reject instance-served responses that outlive their instance

MCP status and skills were cleared on a switch but their in-flight requests
were not, so a response for the previous instance could still write itself over
the new one's — the same race the quota store already guards. Both now carry a
generation.

Usage also claimed an instance as loaded before it had answered, so a load that
failed on a cold or briefly unreachable instance was never attempted again, and
the previous instance's display mode and provider selection survived a switch —
which decided what the new instance was even asked for.

packages/ui/src/stores/useSkillsStore.ts

@@ -198,6 +198,10 @@ const SKILLS_LOAD_CACHE_TTL_MS = 5000;
 const DEFAULT_SKILLS_CACHE_KEY = '__default__';
 const skillsLastLoadedAt = new Map<string, number>();
 const skillsLoadInFlight = new Map<string, Promise<boolean>>();
+// Bumped on every runtime switch. Skills are discovered on the connected
+// instance and cached by directory, which two instances can share, so a load
+// already in flight for the previous instance must not write into the new one.
+let skillsGeneration = 0;
 
 const getSkillsCacheKey = (directory: string | null): string => {
   return directory?.trim() || DEFAULT_SKILLS_CACHE_KEY;
@@ -286,6 +290,7 @@ export const useSkillsStore = create<SkillsStore>()(
         skillDraft: null,
 
         resetForRuntimeSwitch: () => {
+          skillsGeneration += 1;
           skillsLastLoadedAt.clear();
           skillsLoadInFlight.clear();
           set({ skills: [], skillsByDirectory: {}, isLoading: false });
@@ -316,6 +321,7 @@ export const useSkillsStore = create<SkillsStore>()(
             return inFlight;
           }
 
+          const generation = skillsGeneration;
           const request = (async () => {
             set({ isLoading: true });
             // Failure must never look like an empty project. The mirror is the
@@ -361,6 +367,7 @@ export const useSkillsStore = create<SkillsStore>()(
                   data.externalSkills ?? null,
                 );
 
+                if (generation !== skillsGeneration) return false;
                 set((state) => {
                   const next: Partial<SkillsStore> = {
                     skillsByDirectory: { ...state.skillsByDirectory, [cacheKey]: visibleSkills },
@@ -379,6 +386,7 @@ export const useSkillsStore = create<SkillsStore>()(
             }
 
             console.error("Failed to load skills:", lastError);
+            if (generation !== skillsGeneration) return false;
             set((state) => {
               const next: Partial<SkillsStore> = {
                 skillsByDirectory: { ...state.skillsByDirectory, [cacheKey]: previousSkills },