pipeline: add testing improvements for branch-based workflows

Implement dual improvements to enable efficient testing of pipeline changes
without polluting master branch:

1. Add --force flag to create-host script
   - Skip hostname/IP uniqueness validation
   - Overwrite existing host configurations
   - Update entries in flake.nix and terraform/vms.tf (no duplicates)
   - Useful for iterating on configurations during testing

2. Add branch support to bootstrap mechanism
   - Bootstrap service reads NIXOS_FLAKE_BRANCH environment variable
   - Defaults to master if not set
   - Uses branch in git URL via ?ref= parameter
   - Service loads environment from /etc/environment

3. Add cloud-init disk support for branch configuration
   - VMs can specify flake_branch field in terraform/vms.tf
   - Automatically generates cloud-init snippet setting NIXOS_FLAKE_BRANCH
   - Uploads snippet to Proxmox via SSH
   - Production VMs omit flake_branch and use master

4. Update documentation
   - Document --force flag usage in create-host README
   - Add branch testing examples in terraform README
   - Update TODO.md with testing workflow
   - Add .generated/ to gitignore

Testing workflow: Create feature branch, set flake_branch in VM definition,
deploy with terraform, iterate with --force flag, clean up before merging.

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

scripts/create-host/README.md

@@ -50,6 +50,23 @@ python -m scripts.create_host.create_host create \
   --dry-run
 ```
 
+### Force Mode (Regenerate Existing Configuration)
+
+Overwrite an existing host configuration (useful for testing):
+
+```bash
+python -m scripts.create_host.create_host create \
+  --hostname test01 \
+  --ip 10.69.13.50/24 \
+  --force
+```
+
+This mode:
+- Skips hostname and IP uniqueness validation
+- Overwrites files in `hosts/<hostname>/`
+- Updates existing entries in `flake.nix` and `terraform/vms.tf` (doesn't duplicate)
+- Useful for iterating on configuration templates during testing
+
 ### Options
 
 - `--hostname` (required): Hostname for the new host
@@ -73,6 +90,10 @@ python -m scripts.create_host.create_host create \
 
 - `--dry-run` (flag): Preview changes without creating files
 
+- `--force` (flag): Overwrite existing host configuration
+  - Skips uniqueness validation
+  - Updates existing entries instead of creating duplicates
+
 ## What It Does
 
 The tool performs the following actions: