fix: don't block one-shot openclaw subcommands; migrate to current plugin SDK

Bridge server lifecycle:
- Move createBridgeServer() out of register() into an api.on("gateway_start", ...)
  handler. register() runs in every CLI subprocess that loads plugins
  (e.g. `openclaw completion`, `openclaw doctor`); eagerly binding the
  bridge HTTP listener there could pin those processes when no gateway
  is already holding the port.
- Call server.unref() so the listener never pins the host's event loop,
  even if startup somehow runs outside the gateway.

Plugin SDK convention update:
- Wrap default export with definePluginEntry({ id, name, description, register })
  per the current openclaw plugin authoring contract.
- Switch imports from the deprecated root barrel "openclaw/plugin-sdk" to
  focused "openclaw/plugin-sdk/core" / "openclaw/plugin-sdk/plugin-entry".
- Modernize openclaw.plugin.json: drop version/main, add activation.onStartup
  so gateway_start fires for this plugin at boot, declare commandAliases
  for the contractor-agents CLI command.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

plugin/commands/register-cli.ts

@@ -1,4 +1,4 @@
-import type { OpenClawPluginApi } from "openclaw/plugin-sdk";
+import type { OpenClawPluginApi } from "openclaw/plugin-sdk/core";
 import { runContractorAgentsAdd } from "./contractor-agents-add.js";
 
 export function registerCli(api: OpenClawPluginApi): void {