fix(memory): avoid Bun Windows EEXIST on startup memory dir mkdir

[virattt]

Summary

• add a shared ensureDirectory() helper that safely handles Bun-on-Windows mkdir(..., { recursive: true }) throwing EEXIST for already-existing directories
• use this helper in memory initialization/write paths:
  • MemoryStore.ensureDirectoryExists()
  • MemoryStore.writeMemoryFile() parent directory creation
  • MemoryDatabase.create() parent directory creation
• add regression tests for ensureDirectory() behavior when target already exists, is missing, and is a file

Why

Issue #242 reports:

• Error: EEXIST: file already exists, mkdir '.dexter\\memory'

Dexter currently calls recursive mkdir on startup for .dexter/memory. On affected Bun/Windows versions, this can throw EEXIST even when the directory already exists, causing memory initialization to fail before the existing fallback handling.

Validation

• static review of affected call sites completed
• unable to execute bun run typecheck / bun test in this cloud environment because bun is not installed (bun: command not found)

Risk

Low. Changes are limited to directory creation behavior and preserve existing failure semantics for non-directory collisions.