fix: persist scan data on exit and save partial reports (fixes #294)

Two related fixes for reports not being saved when Strix exits:

1. Call tracer.cleanup() in main.py's finally block before posthog.end()
   so that run data is always written to disk regardless of whether the
   atexit handlers registered in cli.py/tui.py fire (they can be skipped
   on Windows when the asyncio event loop tears down).

2. Replace the strict all-or-nothing validation in finish_actions.py with
   a graceful fallback: missing fields get the placeholder
   "[Not provided by model]" so partial reports are saved rather than
   silently discarded when the LLM omits a section.

Co-Authored-By: Octopus <liyuan851277048@icloud.com>

strix/interface/main.py

@@ -629,6 +629,7 @@ def main() -> None:  # noqa: PLR0912, PLR0915
     finally:
         tracer = get_global_tracer()
         if tracer:
+            tracer.cleanup()
             posthog.end(tracer, exit_reason=exit_reason)
 
     results_path = Path("strix_runs") / args.run_name

strix/tools/finish/finish_actions.py

@@ -99,19 +99,11 @@ def finish_scan(
     if active_agents_error:
         return active_agents_error
 
-    validation_errors = []
-
-    if not executive_summary or not executive_summary.strip():
-        validation_errors.append("Executive summary cannot be empty")
-    if not methodology or not methodology.strip():
-        validation_errors.append("Methodology cannot be empty")
-    if not technical_analysis or not technical_analysis.strip():
-        validation_errors.append("Technical analysis cannot be empty")
-    if not recommendations or not recommendations.strip():
-        validation_errors.append("Recommendations cannot be empty")
-
-    if validation_errors:
-        return {"success": False, "message": "Validation failed", "errors": validation_errors}
+    _NOT_PROVIDED = "[Not provided by model]"
+    executive_summary = (executive_summary or "").strip() or _NOT_PROVIDED
+    methodology = (methodology or "").strip() or _NOT_PROVIDED
+    technical_analysis = (technical_analysis or "").strip() or _NOT_PROVIDED
+    recommendations = (recommendations or "").strip() or _NOT_PROVIDED
 
     try:
         from strix.telemetry.tracer import get_global_tracer