🤖 fix: use git show-branch for clearer SSH workspace force delete errors (#530)

## Problem

When attempting to delete SSH workspaces with unpushed commits, the
current error message uses `git log --branches --not --remotes` which
just lists commits without context. This is especially confusing in
squash-merge workflows where feature branch commits never appear in
main, so they always show as "unpushed" even after the PR is merged.

## Solution

Replace the git log command with `git show-branch <current-branch>
origin/<default-branch>` to show the relationship between branches
visually.

**Before:**
```
5d307e9 feat: add feature work 2
5e195dd feat: add feature work 1
```

**After:**
```
Branch status compared to origin/main:

* [feature-branch] feat: add feature work 2
 ! [origin/main] feat: add all feature work (squashed)
--
 * [origin/main] feat: add all feature work (squashed)
+  [feature-branch] feat: add feature work 2
+  [feature-branch^] feat: add feature work 1
+* [origin/main^] Initial commit

Note: If your PR was squash-merged, these commits are already
in origin/main and safe to delete.
```

The `+`, `*`, and `+*` markers clearly show which commits are in which
branch, making it immediately obvious when dealing with a squash-merge
scenario.

## Implementation

- Detects default branch using `git symbolic-ref
refs/remotes/origin/HEAD` with fallbacks to main/master
- Uses `git show-branch` to display branch relationships
- Falls back to original `git log` command when:
  - No remote branches exist
  - Default branch cannot be determined  
  - show-branch fails for any reason

This ensures backward compatibility while providing better UX in the
common case.

## Testing

All existing integration tests pass, including the test that validates
error messages contain commit information.

_Generated with `cmux`_

Author: ammar-agent

src/runtime/SSHRuntime.ts

@@ -985,9 +985,33 @@ export class SSHRuntime implements Runtime {
             cd ${shescape.quote(deletedPath)} || exit 1
             git diff --quiet --exit-code && git diff --quiet --cached --exit-code || exit 1
             if git remote | grep -q .; then
+              # First, check the original condition: any commits not in any remote
               unpushed=$(git log --branches --not --remotes --oneline)
               if [ -n "$unpushed" ]; then
-                echo "$unpushed" | head -10 >&2
+                # 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 "$DEFAULT" ]; then
+                  if git rev-parse --verify origin/main >/dev/null 2>&1; then
+                    DEFAULT="main"
+                  elif git rev-parse --verify origin/master >/dev/null 2>&1; then
+                    DEFAULT="master"
+                  fi
+                fi
+                
+                # If we have both branch and default, use show-branch for better output
+                if [ -n "$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/$DEFAULT and safe to delete." >&2
+                else
+                  # Fallback to just showing the commit list
+                  echo "$unpushed" | head -10 >&2
+                fi
                 exit 2
               fi
             fi