feat: add interactive mode for agent loop

Re-architects the agent loop to support interactive (chat-like) mode
where text-only responses pause execution and wait for user input,
while tool-call responses continue looping autonomously.

- Add `interactive` flag to LLMConfig (default False, no regression)
- Add configurable `waiting_timeout` to AgentState (0 = disabled)
- _process_iteration returns None for text-only → agent_loop pauses
- Conditional system prompt: interactive allows natural text responses
- Skip <meta>Continue the task.</meta> injection in interactive mode
- Sub-agents inherit interactive from parent (300s auto-resume timeout)
- Root interactive agents wait indefinitely for user input (timeout=0)
- TUI sets interactive=True; CLI unchanged (non_interactive=True)

strix/interface/tui.py

@@ -747,7 +747,7 @@ class StrixTUIApp(App):  # type: ignore[misc]
 
     def _build_agent_config(self, args: argparse.Namespace) -> dict[str, Any]:
         scan_mode = getattr(args, "scan_mode", "deep")
-        llm_config = LLMConfig(scan_mode=scan_mode)
+        llm_config = LLMConfig(scan_mode=scan_mode, interactive=True)
 
         config = {
             "llm_config": llm_config,