🐛 fix: wire up readiness cancellation and stabilise graceful shutdown test

Three related issues fixed together:

1. Readiness context was never cancelled during shutdown
   server.Run() had a type assertion for a Cancel() method that no standard
   context.Context implements, so readiness stayed "ready" through the entire
   shutdown window. Added CancelableContext to pkg/server — a thin wrapper that
   exposes Cancel() — and switched cmd/server/main.go to use it. Test servers
   and CLI continue passing context.Background() unchanged.

2. "Server exited" log was never emitted
   The test script expected it; main.go had no log after server.Run() returned.
   Added log.Trace().Msg("Server exited") after the Run() call.

3. Double-SIGTERM caused non-JSON "signal: terminated" in server.log
   test-graceful-shutdown.sh sent SIGTERM, then called $SERVER_CMD stop which
   sent a second SIGTERM. After signal.NotifyContext is cancelled, the second
   signal hits the default handler and Go prints "signal: terminated" to stderr,
   breaking the all-JSON-lines assertion. Fixed by waiting for the PID to exit
   ourselves instead of re-invoking the stop script.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/test-graceful-shutdown.sh

@@ -60,11 +60,39 @@ echo "Response: $GREET_NAME_RESPONSE"
 echo ""
 echo "Stopping server gracefully..."
 
-# Test readiness during shutdown (in background)
-(curl -s http://localhost:8080/api/ready > /dev/null 2>&1 &)
+# Send SIGTERM once and probe /api/ready during the 1-second propagation window
+# the server holds open (pkg/server/server.go: time.Sleep(1s) after readiness
+# cancel). Previously the curl fired *before* the signal — it always saw "ready".
+# We also avoid calling "$SERVER_CMD stop" afterwards because that would send a
+# second SIGTERM: after signal.NotifyContext is done, the default handler kicks in
+# and the process terminates with a non-JSON "signal: terminated" on stderr.
+SERVER_PID=$(cat "$PID_FILE" 2>/dev/null || echo "")
+if [[ -z "$SERVER_PID" ]]; then
+    echo -e "\033[0;31m❌ FAIL: PID file not found\033[0m"
+    exit 1
+fi
 
-$SERVER_CMD stop
-sleep 3
+kill -TERM "$SERVER_PID"
+# Brief yield so the signal handler runs and CancelableContext.Cancel() fires
+sleep 0.2
+curl -s http://localhost:8080/api/ready > /dev/null 2>&1 || true
+
+# Wait for the process to exit cleanly (up to 30s) without sending another signal
+echo "Waiting for server to exit..."
+for i in {1..30}; do
+    if ! ps -p "$SERVER_PID" > /dev/null 2>&1; then
+        echo "Server stopped successfully"
+        rm -f "$PID_FILE"
+        break
+    fi
+    sleep 1
+done
+if ps -p "$SERVER_PID" > /dev/null 2>&1; then
+    echo -e "\033[0;31m❌ FAIL: Server did not stop within 30s\033[0m"
+    kill -9 "$SERVER_PID" 2>/dev/null || true
+    exit 1
+fi
+sleep 0.5
 
 echo ""
 echo "Analyzing server logs..."