- memory_setup.py: use shlex.split() for plugin dep checks instead of shell=True
- transcription_tools.py: avoid shell=True for auto-detected whisper commands
(user-provided templates via env var still use shell=True for compatibility)
- cli.py: add comment clarifying intentional shell=True for user quick_commands
- Add test verifying auto-detected template is shlex-safe
Addresses CONTRIBUTING.md Priority #3 (Security hardening — shell injection).