feat: Implement widget discovery feature in GUI automation

- Added `DiscoverWidgets` RPC to the ImGuiTestHarness service for enumerating GUI widgets.
- Introduced `WidgetDiscoveryService` to handle widget collection and filtering based on various criteria.
- Updated `agent gui discover` command to support new options for filtering and output formats.
- Enhanced `GuiAutomationClient` to facilitate widget discovery requests and responses.
- Added necessary protobuf messages for widget discovery in `imgui_test_harness.proto`.
- Updated CLI command handling to include new GUI discovery functionality.
- Improved documentation for the `agent gui discover` command with examples and output formats.

src/cli/modern_cli.cc

@@ -60,11 +60,13 @@ void ModernCLI::SetupCommands() {
     commands_["agent"] = {
       .name = "agent",
       .description = "Interact with the AI agent",
-      .usage = "z3ed agent <run|plan|diff|test|list|learn|commit|revert|describe> [options]\n"
+      .usage = "z3ed agent <run|plan|diff|test|gui|list|learn|commit|revert|describe> [options]\n"
                "  test run:     --prompt \"<description>\" [--host <host>] [--port <port>] [--timeout <sec>]\n"
                "  test status:  status --test-id <id> [--follow] [--host <host>] [--port <port>]\n"
                "  test list:    list [--category <name>] [--status <state>] [--limit <n>] [--host <host>] [--port <port>]\n"
                "  test results: results --test-id <id> [--include-logs] [--format yaml|json] [--host <host>] [--port <port>]\n"
+               "  gui discover: discover [--window <name>] [--type <widget>] [--path-prefix <path>]\n"
+               "                 [--include-invisible] [--include-disabled] [--format table|json] [--limit <n>]\n"
                "  describe options: [--resource <name>] [--format json|yaml] [--output <path>]\n"
                "                     [--version <value>] [--last-updated <date>]",
       .handler = [this](const std::vector<std::string>& args) -> absl::Status {