Add Troubleshooting

Troubleshooting.md

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+# Troubleshooting
+
+This page collects common failure modes and recovery steps.
+
+## Build Failures
+
+Run with full trace:
+
+```bash
+nix flake check --show-trace
+```
+
+## Remote Builder Issues
+
+Verify SSH access and disk space on the builder.
+
+## LXC Console Issues
+
+If the Proxmox console is blank, type blindly or use SSH.
+
+## Disk Space
+
+Clean the Nix store:
+
+```bash
+nix-collect-garbage -d
+nix store optimise
+```
+
+## Boot Failures
+
+Reboot and select a previous generation from the boot menu.