fix(consistency): correct behind-remote detection to not trigger on local-ahead

_detect_behind_remote was comparing HEAD != @{u} which incorrectly
triggered C-16 when the local repo had unpushed commits. Fixed to use
git rev-list --count HEAD..@{u} which only counts commits the remote
has that local lacks. Adds test_returns_false_when_local_ahead.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/consistency_check.py

@@ -748,8 +748,10 @@ def _check_ghost_duplicates(
 # ---------------------------------------------------------------------------
 
 def _detect_behind_remote(repo_path: str) -> bool:
-    """Return True if the local repo is behind its remote tracking branch.
+    """Return True if the remote tracking branch has commits the local repo lacks.
 
+    "Ahead" (local has unpushed commits) is NOT considered behind.
+    "Diverged" is treated as behind (remote progress could be lost).
     Best-effort: returns False on any error (offline, no remote, etc.) so that
     check-only mode is never blocked by network issues.
     """
@@ -758,15 +760,15 @@ def _detect_behind_remote(repo_path: str) -> bool:
             ["git", "-C", repo_path, "fetch", "--quiet", "origin"],
             capture_output=True, timeout=15,
         )
-        local = subprocess.run(
-            ["git", "-C", repo_path, "rev-parse", "HEAD"],
+        # Count commits in remote that are not in local.
+        # git rev-list HEAD..@{u} → commits remote has that local lacks.
+        result = subprocess.run(
+            ["git", "-C", repo_path, "rev-list", "--count", "HEAD..@{u}"],
             capture_output=True, text=True, timeout=5,
-        ).stdout.strip()
-        remote = subprocess.run(
-            ["git", "-C", repo_path, "rev-parse", "@{u}"],
-            capture_output=True, text=True, timeout=5,
-        ).stdout.strip()
-        return bool(local and remote and local != remote)
+        )
+        if result.returncode != 0:
+            return False  # no upstream configured or other error
+        return int(result.stdout.strip() or "0") > 0
     except Exception:
         return False