feat: add --scan-mode CLI option with quick/standard/deep modes

Introduces scan mode selection to control testing depth and methodology:
- quick: optimized for CI/CD, focuses on recent changes and high-impact vulns
- standard: balanced coverage with systematic methodology
- deep: exhaustive testing with hierarchical agent swarm (now default)

Each mode has dedicated prompt modules with detailed pentesting guidelines
covering reconnaissance, mapping, business logic analysis, exploitation,
and vulnerability chaining strategies.

Closes #152

strix/interface/tui.py

@@ -319,7 +319,8 @@ class StrixTUIApp(App):  # type: ignore[misc]
         }
 
     def _build_agent_config(self, args: argparse.Namespace) -> dict[str, Any]:
-        llm_config = LLMConfig()
+        scan_mode = getattr(args, "scan_mode", "deep")
+        llm_config = LLMConfig(scan_mode=scan_mode)
 
         config = {
             "llm_config": llm_config,