fix: correct release script PR detection using branch name

The script was using fuzzy text search which incorrectly matched unrelated
PRs (e.g., PR #1911, a dependabot PR, was detected as release PR for v0.1.34).
Changed to search by exact branch name (release/vX.Y.Z) which is more reliable.

- Replace --search with --head flag for exact branch matching
- Prevents false positive matches from GitHub's fuzzy search
- Returns empty array when no PR exists (correct behavior)

[AI-assisted debugging and comment]

Author: sanity

scripts/release.sh

@@ -367,7 +367,7 @@ create_release_pr() {
 
     # Check if a release PR for this version already exists or was merged
     echo -n "  Checking for existing release PR... "
-    local existing_pr=$(gh pr list --search "Release $VERSION OR 🚀 Release $VERSION" --state all --limit 1 --json number,state,title --jq '.[] | "\(.number)|\(.state)|\(.title)"' 2>/dev/null || echo "")
+    local existing_pr=$(gh pr list --head "$branch_name" --state all --limit 1 --json number,state,title --jq '.[] | "\(.number)|\(.state)|\(.title)"' 2>/dev/null || echo "")
 
     if [[ -n "$existing_pr" ]]; then
         local pr_number=$(echo "$existing_pr" | cut -d'|' -f1)