fix(ci): flush profraw coverage data in E2E teardown (#550)

The ironclaw binary only handles SIGINT (via tokio::signal::ctrl_c),
not SIGTERM. When conftest.py sent SIGTERM during teardown, the OS
killed the process immediately without running atexit handlers, so
LLVM never flushed .profraw files. cargo llvm-cov report then found
zero profraw files and failed.

- Send SIGINT instead of SIGTERM so the existing ctrl_c handler
  triggers graceful shutdown → main() returns → atexit runs → profraw
  flushed
- Increase shutdown wait from 5s to 10s for graceful cleanup
- Add a diagnostic step to verify profraw files exist before the
  report step, making future issues visible in CI logs

Co-authored-by: Claude Opus 4.6 <[email protected]>

.github/workflows/coverage.yml

@@ -131,6 +131,18 @@ jobs:
           RUST_LOG: ironclaw=info
           RUST_BACKTRACE: "1"
 
+      - name: Verify profraw files exist
+        if: always()
+        run: |
+          echo "LLVM_PROFILE_FILE=${LLVM_PROFILE_FILE}"
+          echo "CARGO_LLVM_COV_TARGET_DIR=${CARGO_LLVM_COV_TARGET_DIR}"
+          profraw_count=$(find target/ -name '*.profraw' 2>/dev/null | wc -l)
+          echo "Found ${profraw_count} .profraw files under target/"
+          find target/ -name '*.profraw' 2>/dev/null || true
+          if [ "$profraw_count" -eq 0 ]; then
+            echo "::warning::No .profraw files found — coverage report will fail"
+          fi
+
       - name: Generate coverage report
         if: always()
         run: cargo llvm-cov report --lcov --output-path e2e-coverage.info