claim_bead: exit 1 on failed ctl write instead of printing success

9p write error was ignored; bots always printed "claimed" regardless
of whether the server rejected the claim with ErrAlreadyClaimed.

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

mcptools/claim_bead.sh

@@ -26,5 +26,8 @@ if [ -z "${AGENT_ID:-}" ]; then
     exit 1
 fi
 
-echo "claim $BEAD_ID $AGENT_ID" | 9p write anvillm/beads/$MOUNT/ctl
+if ! result=$(echo "claim $BEAD_ID $AGENT_ID" | 9p write anvillm/beads/$MOUNT/ctl 2>&1); then
+    echo "error: claim failed: $result" >&2
+    exit 1
+fi
 echo "claimed $BEAD_ID → $AGENT_ID"