docs: document structured output schema

SCHEMA.md

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+# Structured Output Schema
+
+`twitter-cli` uses a shared agent-friendly envelope for machine-readable output.
+
+## Success
+
+```yaml
+ok: true
+schema_version: "1"
+data: ...
+```
+
+## Error
+
+```yaml
+ok: false
+schema_version: "1"
+error:
+  code: api_error
+  message: User @foo not found
+```
+
+## Notes
+
+- `--yaml` and `--json` both use this envelope
+- non-TTY stdout defaults to YAML
+- tweet and user lists are returned under `data`
+- `status` returns `data.authenticated` plus `data.user`
+- `whoami` returns `data.user`