Fix Windows portability: hardcoded /tmp paths

Fixes #227

Summary

Replaces the hardcoded Linux /tmp paths in hexstrike_server.py with portable tempfile.gettempdir()-based defaults, so the server can start and run on Windows (and any host without /tmp).

Changes

• PythonEnvironmentManager.__init__ — default base_dir now resolves to tempfile.gettempdir()/hexstrike_envs (was /tmp/hexstrike_envs, which crashes at import on Windows).
• FileOperationsManager.__init__ — same pattern (hexstrike_files).
• Two screenshot output paths now resolve through tempfile.gettempdir().
• mkdir(..., parents=True) so missing parent dirs are created.
• Added import tempfile.

Why

On Windows the hardcoded /tmp/hexstrike_envs broke module import (see #227); the others failed at runtime.

Testing

• python -m py_compile hexstrike_server.py passes.
• Server starts and serves GET /health on Windows after the change.