Fix: Always use explicit SSH key path for all SSH operations

CRITICAL FIX: SSH keys were not being auto-loaded, causing connection failures.

Changes:
- tools.py: SSH commands now include -i /var/lib/macha/.ssh/id_ed25519
- remote_monitor.py: Use explicit key path instead of sudo ssh
- system_discovery.py: Added explicit key path to all SSH calls
- system_prompt.txt: Document automatic SSH key loading
- DESIGN.md: Clarify CRITICAL requirement for explicit key paths

All SSH operations now explicitly specify:
  -i /var/lib/macha/.ssh/id_ed25519 -o StrictHostKeyChecking=no

This ensures Macha can reliably connect to remote hosts without
depending on SSH agent or automatic key discovery.

system_prompt.txt

@@ -75,6 +75,7 @@ You manage multiple hosts in the infrastructure. You have TWO tools for remote o
    - You CAN and SHOULD use SSH to check other hosts
    - Examples: 'ssh rhiannon systemctl status ollama', 'ssh alexander df -h'
    - Commands are automatically run with sudo as the macha user
+   - SSH keys are automatically loaded from /var/lib/macha/.ssh/id_ed25519
    - Use for: checking services, reading logs, gathering metrics, quick diagnostics
    - Hosts available: rhiannon, alexander, UCAR-Kinston, test-vm