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.

tools/file_operations.py

@@ -819,6 +819,14 @@ class ShellFileOperations(FileOperations):
         # Expand ~ and other shell paths
         path = self._expand_path(path)
         
+        # Validate that the path exists before searching
+        check = self._exec(f"test -e {self._escape_shell_arg(path)} && echo exists || echo not_found")
+        if "not_found" in check.stdout:
+            return SearchResult(
+                error=f"Path not found: {path}. Verify the path exists (use 'terminal' to check).",
+                total_count=0
+            )
+        
         if target == "files":
             return self._search_files(pattern, path, limit, offset)
         else:
@@ -919,6 +927,11 @@ class ShellFileOperations(FileOperations):
         cmd = " ".join(cmd_parts)
         result = self._exec(cmd, timeout=60)
         
+        # rg exit codes: 0=matches found, 1=no matches, 2=error
+        if result.exit_code == 2 and not result.stdout.strip():
+            error_msg = result.stderr.strip() if hasattr(result, 'stderr') and result.stderr else "Search error"
+            return SearchResult(error=f"Search failed: {error_msg}", total_count=0)
+        
         # Parse results based on output mode
         if output_mode == "files_only":
             all_files = [f for f in result.stdout.strip().split('\n') if f]
@@ -1013,6 +1026,11 @@ class ShellFileOperations(FileOperations):
         cmd = " ".join(cmd_parts)
         result = self._exec(cmd, timeout=60)
         
+        # grep exit codes: 0=matches found, 1=no matches, 2=error
+        if result.exit_code == 2 and not result.stdout.strip():
+            error_msg = result.stderr.strip() if hasattr(result, 'stderr') and result.stderr else "Search error"
+            return SearchResult(error=f"Search failed: {error_msg}", total_count=0)
+        
         if output_mode == "files_only":
             all_files = [f for f in result.stdout.strip().split('\n') if f]
             total = len(all_files)