feat: add base templates for all major cloud providers (#26634)

Add 6 new base templates to the template builder, covering all major
cloud providers and platforms:

- **scratch**: Minimal starter template with only `coder_agent` and
metadata
- **aws-windows**: AWS EC2 Windows instances with PowerShell user_data
- **azure-linux**: Azure VMs with cloud-init and managed disk
persistence
- **gcp-linux**: Google Compute Engine Linux instances with persistent
disk
- **gcp-windows**: Google Compute Engine Windows instances
- **digitalocean-linux**: DigitalOcean Linux droplets with persistent
volumes

Each base includes `base.json`, `main.tf.tmpl`, `README.md` (with
prerequisite markers), and any static files (cloud-init configs). Tests
verify all 9 bases load, render without error, and produce valid
single-agent declarations.

`azure-windows` is deferred; it needs to be registered in the `examples`
package first.

Depends on #26633

> [!NOTE]
> This PR was authored by Coder Agents on behalf of @jeremyruppel.

---

NB: This is very much an agent-generated PR and draws completely from
base templates that exist in `examples/templates/`. The base.json files
are new, so review those, but don't spend any brain tokens on the
correctness of the terraform and supporting files: any issues there are
issues with the upstream example template

coderd/templatebuilder/compose_test.go

@@ -78,6 +78,27 @@ func TestCompose(t *testing.T) {
 		require.Contains(t, result.ExtraFiles, "cloud-init/userdata.sh.tftpl")
 	})
 
+	t.Run("GCPWindowsBase", func(t *testing.T) {
+		t.Parallel()
+		result, err := templatebuilder.Compose(templatebuilder.ComposeRequest{
+			BaseTemplateID: "gcp-windows",
+			RegistryURL:    "https://registry.coder.com",
+		})
+		require.NoError(t, err)
+		require.NotEmpty(t, result.MainTF)
+		require.Contains(t, string(result.MainTF), `resource "coder_agent" "main"`)
+	})
+
+	t.Run("AzureLinuxExtraFiles", func(t *testing.T) {
+		t.Parallel()
+		result, err := templatebuilder.Compose(templatebuilder.ComposeRequest{
+			BaseTemplateID: "azure-linux",
+			RegistryURL:    "https://registry.coder.com",
+		})
+		require.NoError(t, err)
+		require.Contains(t, result.ExtraFiles, "cloud-init/cloud-config.yaml.tftpl")
+	})
+
 	t.Run("SensitiveVariable", func(t *testing.T) {
 		t.Parallel()
 		result, err := templatebuilder.Compose(templatebuilder.ComposeRequest{