feat: add twitter article markdown command (#16)

SCHEMA.md

@@ -25,10 +25,24 @@ error:
 - `--yaml` and `--json` both use this envelope
 - non-TTY stdout defaults to YAML
 - tweet and user lists are returned under `data`
+- `article` returns a single tweet object under `data`
 - `status` returns `data.authenticated` plus `data.user`
 - `whoami` returns `data.user`
 - write commands also support explicit `--json` / `--yaml`
 
+## Article Fields
+
+`twitter article <id> --json` returns the standard tweet object plus:
+
+```yaml
+data:
+  id: "1234567890"
+  articleTitle: "Article Title"
+  articleText: |
+    # Heading
+    Body text...
+```
+
 ## Error Codes
 
 Common structured error codes: