fix: prefer main/master over origin/HEAD for squash detection

When a bare repo is cloned from a feature branch, origin/HEAD points
to that feature branch. This caused the squash-merge detection to
compare against the wrong branch.

Now we prefer origin/main or origin/master over origin/HEAD since
those are almost always the actual default branches for squash merges.

Author: ammar-agent

src/node/runtime/SSHRuntime.ts

@@ -1087,14 +1087,15 @@ export class SSHRuntime implements Runtime {
                 # Get current branch for better error messaging
                 BRANCH=$(git rev-parse --abbrev-ref HEAD 2>/dev/null)
                 
-                # Get default branch (try origin/HEAD, fallback to main, then master)
-                DEFAULT=$(git symbolic-ref refs/remotes/origin/HEAD 2>/dev/null | sed 's@^refs/remotes/origin/@@')
-                if [ -z "$DEFAULT" ]; then
-                  if git rev-parse --verify origin/main >/dev/null 2>&1; then
-                    DEFAULT="main"
-                  elif git rev-parse --verify origin/master >/dev/null 2>&1; then
-                    DEFAULT="master"
-                  fi
+                # Get default branch (prefer main/master over origin/HEAD since origin/HEAD
+                # might point to a feature branch in some setups)
+                if git rev-parse --verify origin/main >/dev/null 2>&1; then
+                  DEFAULT="main"
+                elif git rev-parse --verify origin/master >/dev/null 2>&1; then
+                  DEFAULT="master"
+                else
+                  # Fallback to origin/HEAD if main/master don't exist
+                  DEFAULT=$(git symbolic-ref refs/remotes/origin/HEAD 2>/dev/null | sed 's@^refs/remotes/origin/@@')
                 fi
                 
                 # Check for squash-merge: if all changed files match origin/$DEFAULT, content is merged