refactor(shared): unify MESSAGE_STORAGE/PART_STORAGE constants into single source

- Create src/shared/opencode-storage-paths.ts with all 4 constants
- Update 4 previous declaration sites to import from shared file
- Update additional OPENCODE_STORAGE usages for consistency
- Re-export from src/shared/index.ts
- No duplicate constant declarations remain

src/shared/opencode-message-dir.ts

@@ -1,11 +1,9 @@
 import { existsSync, readdirSync } from "node:fs"
 import { join } from "node:path"
-import { getOpenCodeStorageDir } from "./data-path"
+import { MESSAGE_STORAGE } from "./opencode-storage-paths"
 import { isSqliteBackend } from "./opencode-storage-detection"
 import { log } from "./logger"
 
-const MESSAGE_STORAGE = join(getOpenCodeStorageDir(), "message")
-
 export function getMessageDir(sessionID: string): string | null {
   if (!sessionID.startsWith("ses_")) return null
   if (isSqliteBackend()) return null