Add Scarf telemetry alongside PostHog

Both backends share session/version/first-run helpers in
strix/telemetry/_common.py and fire from the same four call sites in
strix/interface/main.py and strix/report/state.py. STRIX_TELEMETRY is
the single toggle for both.

strix/report/state.py

@@ -16,7 +16,7 @@ from strix.report.writer import (
     write_run_record,
     write_vulnerabilities,
 )
-from strix.telemetry import posthog
+from strix.telemetry import posthog, scarf
 
 
 logger = logging.getLogger(__name__)
@@ -202,6 +202,7 @@ class ReportState:
         self.vulnerability_reports.append(report)
         logger.info(f"Added vulnerability report: {report_id} - {title}")
         posthog.finding(severity)
+        scarf.finding(severity)
 
         if self.vulnerability_found_callback:
             self.vulnerability_found_callback(report)
@@ -254,6 +255,7 @@ class ReportState:
         logger.info("Updated scan final fields")
         self.save_run_data(mark_complete=True)
         posthog.end(self, exit_reason="finished_by_tool")
+        scarf.end(self, exit_reason="finished_by_tool")
 
     def set_scan_config(self, config: dict[str, Any]) -> None:
         self.scan_config = config