fix: search_files now reports error for non-existent paths instead of silent empty results

Previously, search_files would silently return 0 results when the
search path didn't exist (e.g., /root/.hermes/... when HOME is
/home/user). The path was passed to rg/grep/find which would fail
silently, and the empty stdout was parsed as 'no matches found'.

Changes:
- Add path existence check at the top of search() using test -e.
  Returns SearchResult with a clear error message when path doesn't exist.
- Add exit code 2 checks in _search_with_rg() and _search_with_grep()
  as secondary safety net for other error types (bad regex, permissions).
- Add 4 new tests covering: nonexistent path (content mode), nonexistent
  path (files mode), existing path proceeds normally, rg error exit code.

Tests: 37 → 41 in test_file_operations.py, full suite 2330 passed.

tests/tools/test_file_operations.py

@@ -259,6 +259,70 @@ class TestShellFileOpsHelpers:
         assert ops.cwd == "/"
 
 
+class TestSearchPathValidation:
+    """Test that search() returns an error for non-existent paths."""
+
+    def test_search_nonexistent_path_returns_error(self, mock_env):
+        """search() should return an error when the path doesn't exist."""
+        def side_effect(command, **kwargs):
+            if "test -e" in command:
+                return {"output": "not_found", "returncode": 1}
+            if "command -v" in command:
+                return {"output": "yes", "returncode": 0}
+            return {"output": "", "returncode": 0}
+        mock_env.execute.side_effect = side_effect
+        ops = ShellFileOperations(mock_env)
+        result = ops.search("pattern", path="/nonexistent/path")
+        assert result.error is not None
+        assert "not found" in result.error.lower() or "Path not found" in result.error
+
+    def test_search_nonexistent_path_files_mode(self, mock_env):
+        """search(target='files') should also return error for bad paths."""
+        def side_effect(command, **kwargs):
+            if "test -e" in command:
+                return {"output": "not_found", "returncode": 1}
+            if "command -v" in command:
+                return {"output": "yes", "returncode": 0}
+            return {"output": "", "returncode": 0}
+        mock_env.execute.side_effect = side_effect
+        ops = ShellFileOperations(mock_env)
+        result = ops.search("*.py", path="/nonexistent/path", target="files")
+        assert result.error is not None
+        assert "not found" in result.error.lower() or "Path not found" in result.error
+
+    def test_search_existing_path_proceeds(self, mock_env):
+        """search() should proceed normally when the path exists."""
+        def side_effect(command, **kwargs):
+            if "test -e" in command:
+                return {"output": "exists", "returncode": 0}
+            if "command -v" in command:
+                return {"output": "yes", "returncode": 0}
+            # rg returns exit 1 (no matches) with empty output
+            return {"output": "", "returncode": 1}
+        mock_env.execute.side_effect = side_effect
+        ops = ShellFileOperations(mock_env)
+        result = ops.search("pattern", path="/existing/path")
+        assert result.error is None
+        assert result.total_count == 0  # No matches but no error
+
+    def test_search_rg_error_exit_code(self, mock_env):
+        """search() should report error when rg returns exit code 2."""
+        call_count = {"n": 0}
+        def side_effect(command, **kwargs):
+            call_count["n"] += 1
+            if "test -e" in command:
+                return {"output": "exists", "returncode": 0}
+            if "command -v" in command:
+                return {"output": "yes", "returncode": 0}
+            # rg returns exit 2 (error) with empty output
+            return {"output": "", "returncode": 2}
+        mock_env.execute.side_effect = side_effect
+        ops = ShellFileOperations(mock_env)
+        result = ops.search("pattern", path="/some/path")
+        assert result.error is not None
+        assert "search failed" in result.error.lower() or "Search error" in result.error
+
+
 class TestShellFileOpsWriteDenied:
     def test_write_file_denied_path(self, file_ops):
         result = file_ops.write_file("~/.ssh/authorized_keys", "evil key")