docs: add memory ballooning as fallback option

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/plans/memory-issues-follow-up.md

@@ -90,5 +90,27 @@ After a few days (allow auto-upgrades to run on all hosts):
 If zram is insufficient:
 
 1. **Increase VM memory** - Update `terraform/vms.tf` to 4GB for affected hosts
-2. **Use remote builds** - Configure `nix.buildMachines` to offload evaluation
+2. **Enable memory ballooning** - Configure VMs with dynamic memory allocation (see below)
-3. **Reduce flake size** - Split configurations to reduce evaluation memory
+3. **Use remote builds** - Configure `nix.buildMachines` to offload evaluation
+4. **Reduce flake size** - Split configurations to reduce evaluation memory
+
+### Memory Ballooning
+
+Proxmox supports memory ballooning, which allows VMs to dynamically grow/shrink memory allocation based on demand. The balloon driver inside the guest communicates with the hypervisor to release or reclaim memory pages.
+
+Configuration in `terraform/vms.tf`:
+```hcl
+memory  = 4096  # maximum memory
+balloon = 2048  # minimum memory (shrinks to this when idle)
+```
+
+Pros:
+- VMs get memory on-demand without reboots
+- Better host memory utilization
+- Solves upgrade OOM without permanently allocating 4GB
+
+Cons:
+- Requires QEMU guest agent running in guest
+- Guest can experience memory pressure if host is overcommitted
+
+Ballooning and zram are complementary - ballooning provides headroom from the host, zram provides overflow within the guest.