v0.8.5 - hooks mine_dir NameError fix

Bug Fixes

• hooks/engine.py:_action_mine_dir raised NameError: name 'vault' is not defined on every mine_dir hook execution — it assigned the configured vault to vault_cfg but passed the undefined vault to engine.mine(). Fixed to use vault_cfg.
• Added regression suite tests/test_hooks_engine_mine_dir.py (5 cases, stubbed VaultEngine) covering default vault, custom vault from action config, missing-path skip, nonexistent-path skip, and directory-mode mining.