refactor(coderd/templatebuilder): address review feedback

- Replace stub module with real code-server module manifest
- Export ParseModulesFromFS(fs.FS) for test isolation (no sync.Once coupling)
- Use sync.OnceValues for the cached loader
- Validate module ID (non-empty, unique), pinned_version, and variable types
  during parsing; reject unknown types at load time
- Normalize nil slices to empty in ToSDK() to prevent null in JSON responses
- Distinguish fs.ErrNotExist from other ReadFile errors
- Improve error messages to describe what failed, not internal plumbing
- Rewrite tests with fstest.MapFS fixtures covering all variable types,
  default pointer, validation errors, and full SDK field assertions
- Trim zero-value godoc comments
- Add future tense to README for unimplemented behavior; link RFC

coderd/templatebuilder/modules/code-server/module.json

@@ -0,0 +1,30 @@
+{
+  "id": "code-server",
+  "display_name": "code-server",
+  "description": "VS Code in the browser",
+  "icon": "/icon/code.svg",
+  "category": "IDE",
+  "tags": ["ide", "web"],
+  "compatible_os": ["linux"],
+  "conflicts_with": [],
+  "pinned_version": "1.2.3",
+  "variables": [
+    {
+      "name": "agent_id",
+      "type": "string",
+      "description": "The ID of the Coder agent. Injected automatically by the builder.",
+      "required": true,
+      "sensitive": false,
+      "builder_managed": true
+    },
+    {
+      "name": "port",
+      "type": "number",
+      "description": "Port to run code-server on",
+      "default": "13337",
+      "required": false,
+      "sensitive": false,
+      "builder_managed": false
+    }
+  ]
+}