fix(data-path): writable directory fallback for data/cache paths

getDataDir() and getCacheDir() now verify the directory is writable and
fall back to os.tmpdir() if not.

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/shared/data-path.ts

@@ -1,5 +1,18 @@
 import * as path from "node:path"
 import * as os from "node:os"
+import { accessSync, constants, mkdirSync } from "node:fs"
+
+function resolveWritableDirectory(preferredDir: string, fallbackSuffix: string): string {
+  try {
+    mkdirSync(preferredDir, { recursive: true })
+    accessSync(preferredDir, constants.W_OK)
+    return preferredDir
+  } catch {
+    const fallbackDir = path.join(os.tmpdir(), fallbackSuffix)
+    mkdirSync(fallbackDir, { recursive: true })
+    return fallbackDir
+  }
+}
 
 /**
  * Returns the user-level data directory.
@@ -10,7 +23,8 @@ import * as os from "node:os"
  * including Windows, so we match that behavior exactly.
  */
 export function getDataDir(): string {
-  return process.env.XDG_DATA_HOME ?? path.join(os.homedir(), ".local", "share")
+  const preferredDir = process.env.XDG_DATA_HOME ?? path.join(os.homedir(), ".local", "share")
+  return resolveWritableDirectory(preferredDir, "opencode-data")
 }
 
 /**
@@ -27,7 +41,8 @@ export function getOpenCodeStorageDir(): string {
  * - All platforms: XDG_CACHE_HOME or ~/.cache
  */
 export function getCacheDir(): string {
-  return process.env.XDG_CACHE_HOME ?? path.join(os.homedir(), ".cache")
+  const preferredDir = process.env.XDG_CACHE_HOME ?? path.join(os.homedir(), ".cache")
+  return resolveWritableDirectory(preferredDir, "opencode-cache")
 }
 
 /**