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/azure-linux/cloud-init/cloud-config.yaml.tftpl

@@ -0,0 +1,56 @@
+#cloud-config
+cloud_final_modules:
+- [scripts-user, always]
+bootcmd:
+  # work around https://github.com/hashicorp/terraform-provider-azurerm/issues/6117
+  - until [ -e /dev/disk/azure/scsi1/lun10 ]; do sleep 1; done
+device_aliases:
+  homedir: /dev/disk/azure/scsi1/lun10
+disk_setup:
+  homedir:
+    table_type: gpt
+    layout: true
+fs_setup:
+  - label: coder_home
+    filesystem: ext4
+    device: homedir.1
+mounts:
+  - ["LABEL=coder_home", "/home/${username}"]
+hostname: ${hostname}
+users:
+  - name: ${username}
+    sudo: ["ALL=(ALL) NOPASSWD:ALL"]
+    groups: sudo
+    shell: /bin/bash
+packages:
+  - git
+write_files:
+  - path: /opt/coder/init
+    permissions: "0755"
+    encoding: b64
+    content: ${init_script}
+  - path: /etc/systemd/system/coder-agent.service
+    permissions: "0644"
+    content: |
+      [Unit]
+      Description=Coder Agent
+      After=network-online.target
+      Wants=network-online.target
+
+      [Service]
+      User=${username}
+      ExecStart=/opt/coder/init
+      Restart=always
+      RestartSec=10
+      TimeoutStopSec=90
+      KillMode=process
+
+      OOMScoreAdjust=-900
+      SyslogIdentifier=coder-agent
+
+      [Install]
+      WantedBy=multi-user.target
+runcmd:
+  - chown ${username}:${username} /home/${username}
+  - systemctl enable coder-agent
+  - systemctl start coder-agent