fix(coderd/templatebuilder): fix archive bundling for nested static files and counted agents (#26901)

Fixes two template builder bugs that caused AWS EC2 (Linux) template
imports to fail:

1. **Missing directory entries in tar archive**: `BundleTar` wrote
static files with nested paths (e.g.
`cloud-init/cloud-config.yaml.tftpl`) without emitting `TypeDir` entries
for parent directories. The provisioner's archive extractor requires
explicit directory entries and failed with "no such file or directory".

2. **Incorrect agent reference for counted resources**:
`ExtractAgentResourceName` returned `dev` for the AWS Linux base
template, but the agent uses `count =
data.coder_workspace.me.start_count`, so module templates need
`coder_agent.dev[0].id`. The function now detects `count`/`for_each` and
appends `[0]`.

> [!NOTE]
> Generated by Coder Agents (on behalf of @jeremyruppel)

coderd/templatebuilder/render_test.go

@@ -215,7 +215,28 @@ func TestExtractAgentResourceName(t *testing.T) {
 
 		name, err := templatebuilder.ExtractAgentResourceName(rendered)
 		require.NoError(t, err)
-		require.Equal(t, "dev", name)
+		require.Equal(t, "dev[0]", name, "counted agent should include index")
+	})
+
+	t.Run("CountedAgent", func(t *testing.T) {
+		t.Parallel()
+		hcl := []byte(`resource "coder_agent" "myagent" {
+  count = data.coder_workspace.me.start_count
+  arch  = "amd64"
+}`)
+		name, err := templatebuilder.ExtractAgentResourceName(hcl)
+		require.NoError(t, err)
+		require.Equal(t, "myagent[0]", name)
+	})
+
+	t.Run("UncountedAgent", func(t *testing.T) {
+		t.Parallel()
+		hcl := []byte(`resource "coder_agent" "main" {
+  arch = "amd64"
+}`)
+		name, err := templatebuilder.ExtractAgentResourceName(hcl)
+		require.NoError(t, err)
+		require.Equal(t, "main", name)
 	})
 
 	t.Run("NoAgent", func(t *testing.T) {