fix: detect squash-merged branches for SSH workspace deletion

SSH workspaces required force deletion after squash-merging PRs because
the check used 'git log --branches --not --remotes' which always shows
commits for squash-merged branches (original commits differ from squash
commit SHA).

Added content-based comparison when unpushed commits are detected:
- Fetch latest default branch from origin
- Get files changed on branch since merge-base
- Compare each file's content between HEAD and origin/$DEFAULT
- If all files match, treat as merged (squash-merge case)

This allows clean deletion of squash-merged branches without forcing,
while still protecting branches with genuinely unmerged work.

Author: ammar-agent

src/node/runtime/SSHRuntime.ts

@@ -1097,13 +1097,48 @@ export class SSHRuntime implements Runtime {
                   fi
                 fi
                 
-                # If we have both branch and default, use show-branch for better output
+                # Check for squash-merge: if all changed files match origin/$DEFAULT, content is merged
+                if [ -n "$DEFAULT" ]; then
+                  # Fetch latest to ensure we have current remote state
+                  git fetch origin "$DEFAULT" --quiet 2>/dev/null || true
+                  
+                  # Get merge-base between current branch and default
+                  MERGE_BASE=$(git merge-base "origin/$DEFAULT" HEAD 2>/dev/null)
+                  if [ -n "$MERGE_BASE" ]; then
+                    # Get files changed on this branch since fork point
+                    CHANGED_FILES=$(git diff --name-only "$MERGE_BASE" HEAD 2>/dev/null)
+                    
+                    if [ -n "$CHANGED_FILES" ]; then
+                      # Check if all changed files match what's in origin/$DEFAULT
+                      ALL_MERGED=true
+                      while IFS= read -r f; do
+                        # Compare file content between HEAD and origin/$DEFAULT
+                        # If file doesn't exist in one but exists in other, they differ
+                        if ! git diff --quiet "HEAD:$f" "origin/$DEFAULT:$f" 2>/dev/null; then
+                          ALL_MERGED=false
+                          break
+                        fi
+                      done <<< "$CHANGED_FILES"
+                      
+                      if $ALL_MERGED; then
+                        # All changes are in default branch - safe to delete (squash-merge case)
+                        exit 0
+                      fi
+                    else
+                      # No changed files means nothing to merge - safe to delete
+                      exit 0
+                    fi
+                  fi
+                fi
+                
+                # If we get here, there are real unpushed changes
+                # Show helpful output for debugging
                 if [ -n "$BRANCH" ] && [ -n "$DEFAULT" ] && git show-branch "$BRANCH" "origin/$DEFAULT" >/dev/null 2>&1; then
                   echo "Branch status compared to origin/$DEFAULT:" >&2
                   echo "" >&2
                   git show-branch "$BRANCH" "origin/$DEFAULT" 2>&1 | head -20 >&2
                   echo "" >&2
-                  echo "Note: If your PR was squash-merged, these commits are already in origin/$DEFAULT and safe to delete." >&2
+                  echo "Note: Branch has changes not yet in origin/$DEFAULT." >&2
                 else
                   # Fallback to just showing the commit list
                   echo "$unpushed" | head -10 >&2