fix(infra): fix systemd service node paths and remove unused bin/fabric

- Removed empty bin/fabric file (not used; package.json bin declaration is correct)
- Updated fabric-web.service and fabric-prune.service to use /home/coding/.nix-profile/bin/node instead of /usr/bin/node (NixOS node path)
- Created ~/.config/fabric/secrets.env with FABRIC_AUTH_TOKEN
- Installed and enabled fabric-web.service and fabric-prune.timer

Acceptance verified:
- systemctl --user status fabric-web.service shows active (running)
- curl http://localhost:3000/api/workers returns valid JSON ([])

Closes: bf-1nah

scripts/fabric-prune.service

@@ -5,7 +5,7 @@ After=network.target
 [Service]
 Type=oneshot
 WorkingDirectory=/home/coding/FABRIC
-ExecStart=/usr/bin/node dist/cli.js prune --source ~/.needle/logs
+ExecStart=/home/coding/.nix-profile/bin/node dist/cli.js prune --source ~/.needle/logs
 Environment=NODE_ENV=production
 StandardOutput=journal
 StandardError=journal

scripts/fabric-web.service

@@ -6,7 +6,7 @@ After=network.target
 Type=simple
 WorkingDirectory=/home/coding/FABRIC
 # Run with 1GB heap limit, enable heap snapshots for leak detection
-ExecStart=/usr/bin/node --max-old-space-size=1024 dist/cli.js web --port 3000 --source /home/coding/.needle/logs --otlp-http :4318 --heap-snapshots --snapshot-interval 30
+ExecStart=/home/coding/.nix-profile/bin/node --max-old-space-size=1024 dist/cli.js web --port 3000 --source /home/coding/.needle/logs --otlp-http :4318 --heap-snapshots --snapshot-interval 30
 Restart=on-failure
 RestartSec=5
 # Rate limit restarts: 5 times within 2 minutes before entering failed state