docs: add JSON output example for diagnostic verbs post-#127

USAGE.md now documents:
-  for machine-readable diagnostics
- Note about parse-time rejection of invalid suffix args (post-#127 fix)

Verifies that diagnostic verbs support JSON output for scripting,
and documents the behavior change from #127 (invalid args rejected
at parse time instead of falling through to prompt dispatch).

Refs: #127

Author: code-yeongyu

USAGE.md

@@ -43,6 +43,15 @@ cd rust
 /doctor
 ```
 
+Or run doctor directly with JSON output for scripting:
+
+```bash
+cd rust
+./target/debug/claw doctor --output-format json
+```
+
+**Note:** Diagnostic verbs (`doctor`, `status`, `sandbox`, `version`) support `--output-format json` for machine-readable output. Invalid suffix arguments (e.g., `--json`) are now rejected at parse time rather than falling through to prompt dispatch.
+
 ### Interactive REPL
 
 ```bash