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/render_test.go

@@ -270,6 +270,24 @@ func TestModuleTemplateFS(t *testing.T) {
 	})
 }
 
+func TestAllBasesRenderAndExtractAgent(t *testing.T) {
+	t.Parallel()
+
+	for _, id := range templatebuilder.BaseTemplateIDs() {
+		t.Run(id, func(t *testing.T) {
+			t.Parallel()
+			renderCtx := templatebuilder.DefaultBaseRenderContext(id)
+			rendered, err := templatebuilder.RenderBaseTemplate(id, "main.tf.tmpl", renderCtx)
+			require.NoError(t, err, "base %q should render without error", id)
+			require.NotEmpty(t, rendered)
+
+			name, err := templatebuilder.ExtractAgentResourceName(rendered)
+			require.NoError(t, err, "base %q should have exactly one coder_agent", id)
+			require.NotEmpty(t, name)
+		})
+	}
+}
+
 func TestBaseTemplateSnapshot(t *testing.T) {
 	t.Parallel()
 
@@ -279,6 +297,12 @@ func TestBaseTemplateSnapshot(t *testing.T) {
 		{exampleID: "docker"},
 		{exampleID: "kubernetes"},
 		{exampleID: "aws-linux"},
+		{exampleID: "aws-windows"},
+		{exampleID: "azure-linux"},
+		{exampleID: "digitalocean-linux"},
+		{exampleID: "gcp-linux"},
+		{exampleID: "gcp-windows"},
+		{exampleID: "scratch"},
 	}
 
 	for _, tc := range tests {