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/hooks/interactive-bash-session/constants.ts

@@ -1,7 +1,5 @@
 import { join } from "node:path";
-import { getOpenCodeStorageDir } from "../../shared/data-path";
-
-export const OPENCODE_STORAGE = getOpenCodeStorageDir();
+import { OPENCODE_STORAGE } from "../../shared";
 export const INTERACTIVE_BASH_SESSION_STORAGE = join(
   OPENCODE_STORAGE,
   "interactive-bash-session",