fix(agent): prevent Claude runtime pings from hanging after the model has already finished

Claude's stream-json flow can emit the terminal result event while the
child process still waits on open stdin. Closing stdin as soon as the
final result arrives lets the CLI exit cleanly instead of idling until
the daemon timeout fires.

Constraint: Must preserve the existing Claude stream-json protocol and child-process lifecycle
Rejected: Increase ping timeout only | masks the hang without fixing process exit
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep Claude stdin handling aligned with the stream-json terminal result semantics; do not defer closure until goroutine teardown
Tested: Reproduced self-hosted runtime ping timeout locally; verified ping succeeds after closing stdin on result; cd server && go test ./pkg/agent
Not-tested: Full make check; Bedrock/Vertex-specific Claude auth flows

server/pkg/agent/claude.go

@@ -52,14 +52,21 @@ func (b *claudeBackend) Execute(ctx context.Context, prompt string, opts ExecOpt
 		cancel()
 		return nil, fmt.Errorf("claude stdin pipe: %w", err)
 	}
+	closeStdin := func() {
+		if stdin != nil {
+			_ = stdin.Close()
+			stdin = nil
+		}
+	}
 	cmd.Stderr = newLogWriter(b.cfg.Logger, "[claude:stderr] ")
 
 	if err := cmd.Start(); err != nil {
+		closeStdin()
 		cancel()
 		return nil, fmt.Errorf("start claude: %w", err)
 	}
 	if err := writeClaudeInput(stdin, prompt); err != nil {
-		_ = stdin.Close()
+		closeStdin()
 		cancel()
 		_ = cmd.Wait()
 		return nil, fmt.Errorf("write claude input: %w", err)
@@ -70,7 +77,7 @@ func (b *claudeBackend) Execute(ctx context.Context, prompt string, opts ExecOpt
 	resCh := make(chan Result, 1)
 
 	go func() {
-		defer stdin.Close()
+		defer closeStdin()
 		defer cancel()
 		defer close(msgCh)
 		defer close(resCh)
@@ -107,6 +114,7 @@ func (b *claudeBackend) Execute(ctx context.Context, prompt string, opts ExecOpt
 				}
 				trySend(msgCh, Message{Type: MessageStatus, Status: "running"})
 			case "result":
+				closeStdin()
 				sessionID = msg.SessionID
 				if msg.ResultText != "" {
 					output.Reset()