Fix merge script returning exit 1 on successful merge.

The print_summary function used `[[ -n "$VAR" ]] && echo` for optional
output lines. When the variable was empty, the `[[ ]]` returned 1 and
short-circuited — making 1 the script's exit code despite success.
Replaced with if/then to avoid leaking exit codes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: drn

agents/skills/merge/scripts/merge.sh

@@ -211,8 +211,8 @@ print_summary() {
   echo "pr:       ${PR_URL}"
   echo "branch:   ${BRANCH} → master"
   echo "commits:  ${COMMIT_COUNT}"
-  [[ -n "${MASTER_COMMIT:-}" ]] && echo "commit:   ${MASTER_COMMIT}"
-  [[ -n "${DOTS_SYNCED:-}" ]] && echo "~/.dots:  synced → ${DOTS_SYNCED}"
+  if [[ -n "${MASTER_COMMIT:-}" ]]; then echo "commit:   ${MASTER_COMMIT}"; fi
+  if [[ -n "${DOTS_SYNCED:-}" ]]; then echo "~/.dots:  synced → ${DOTS_SYNCED}"; fi
 }
 
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