fix: use in-process flag instead of existsSync for cache availability

In OpenCode plugin sandbox, existsSync fails across different hook
contexts (config vs chat.message) because each runs in an isolated
virtual filesystem. Add writtenInCurrentProcess flags to track cache
writes at the process level, eliminating spurious Model Cache Not
Found toast warnings.

Closes #3412

src/shared/json-file-cache-store.ts

@@ -27,6 +27,7 @@ export function createJsonFileCacheStore<TValue>(
 	options: JsonFileCacheStoreOptions<TValue>,
 ): JsonFileCacheStore<TValue> {
 	let memoryValue: TValue | null | undefined
+	let writtenInCurrentProcess = false
 
 	function getCacheFilePath(): string {
 		return join(options.getCacheDir(), options.filename)
@@ -67,6 +68,17 @@ export function createJsonFileCacheStore<TValue>(
 	}
 
 	function has(): boolean {
+		// First check if we have a valid in-memory cache value
+		// This handles sandbox environments where existsSync may fail across contexts
+		if (memoryValue !== undefined && memoryValue !== null) {
+			return true
+		}
+		// Check if we've written to this cache in the current process
+		// This helps in sandbox environments where filesystem state may not persist across contexts
+		if (writtenInCurrentProcess) {
+			return true
+		}
+		// Fall back to filesystem check
 		return existsSync(getCacheFilePath())
 	}
 
@@ -77,6 +89,7 @@ export function createJsonFileCacheStore<TValue>(
 		try {
 			writeFileSync(cacheFile, options.serialize?.(value) ?? JSON.stringify(value, null, 2))
 			memoryValue = value
+			writtenInCurrentProcess = true
 			log(`[${options.logPrefix}] ${options.cacheLabel} written`, options.describe(value))
 		} catch (error) {
 			log(`[${options.logPrefix}] Error writing ${toLogLabel(options.cacheLabel)}`, {
@@ -87,6 +100,7 @@ export function createJsonFileCacheStore<TValue>(
 
 	function resetMemory(): void {
 		memoryValue = undefined
+		writtenInCurrentProcess = false
 	}
 
 	return {