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/bases/scratch/README.md

@@ -0,0 +1,32 @@
+---
+display_name: Scratch
+description: A minimal Coder workspace template with just an agent
+icon: ../../../site/static/icon/coder.svg
+maintainer_github: coder
+verified: true
+tags: [minimal, scratch]
+---
+
+# Scratch Template
+
+A minimal template that provisions a Coder agent with basic metadata. Use this as a starting point when you want full control over the infrastructure and just need the agent scaffolding.
+
+<!-- prerequisites:start -->
+
+## Prerequisites
+
+This template only provisions a Coder agent. You must provide your own compute platform (e.g. Docker, a VM, Kubernetes) for the agent to run on.
+
+<!-- prerequisites:end -->
+
+## Architecture
+
+This template provisions the following resources:
+
+- Coder agent (with CPU and RAM metadata)
+- Environment variables for Git configuration
+
+No infrastructure resources are included. Extend the template with your own compute resources.
+
+> **Note**
+> This template is designed to be a starting point! Edit the Terraform to extend the template to support your use case.

coderd/templatebuilder/bases/scratch/base.json

@@ -0,0 +1,6 @@
+{
+	"id": "scratch",
+	"display_name": "Scratch",
+	"os": "linux",
+	"default_context": {}
+}

coderd/templatebuilder/bases/scratch/main.tf.tmpl

@@ -0,0 +1,40 @@
+terraform {
+  required_providers {
+    coder = {
+      source = "coder/coder"
+    }
+  }
+}
+
+data "coder_provisioner" "me" {}
+
+data "coder_workspace" "me" {}
+
+resource "coder_agent" "main" {
+  arch = data.coder_provisioner.me.arch
+  os   = data.coder_provisioner.me.os
+
+  metadata {
+    display_name = "CPU Usage"
+    key          = "0_cpu_usage"
+    script       = "coder stat cpu"
+    interval     = 10
+    timeout      = 1
+  }
+
+  metadata {
+    display_name = "RAM Usage"
+    key          = "1_ram_usage"
+    script       = "coder stat mem"
+    interval     = 10
+    timeout      = 1
+  }
+}
+
+# Use this to set environment variables in your workspace
+# details: https://registry.terraform.io/providers/coder/coder/latest/docs/resources/env
+resource "coder_env" "welcome_message" {
+  agent_id = coder_agent.main.id
+  name     = "WELCOME_MESSAGE"
+  value    = "Welcome to your Coder workspace!"
+}