🐛 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>

cmd/server/main.go

@@ -48,8 +48,10 @@ func main() {
 		log.Fatal().Err(err).Msg("Failed to load configuration")
 	}
 
-	// Create readiness context to control readiness state
-	readyCtx, readyCancel := context.WithCancel(context.Background())
+	// Create readiness context to control readiness state.
+	// CancelableContext exposes Cancel() so that Server.Run() can cancel
+	// readiness at the start of graceful shutdown (before the propagation sleep).
+	readyCtx, readyCancel := server.NewCancelableContext(context.Background())
 	defer readyCancel()
 
 	// Create and run server
@@ -57,4 +59,5 @@ func main() {
 	if err := server.Run(); err != nil {
 		log.Fatal().Err(err).Msg("Server failed")
 	}
+	log.Trace().Msg("Server exited")
 }