feat: add Grok Build plugin and marketplace support

Ship .grok-plugin manifests so Grok can install ECC as a native plugin.
Resolve GROK_PLUGIN_ROOT and ~/.grok plugin homes so trusted hooks run.
Document trust/enable, ECC_HOOK_PROFILE, and root .mcp.json attachment.

tests/scripts/build-opencode.test.js

@@ -324,6 +324,14 @@ function main() {
         packagedPaths.has(".codex-plugin/plugin.json"),
         "npm pack should include .codex-plugin/plugin.json"
       )
+      assert.ok(
+        packagedPaths.has(".grok-plugin/plugin.json"),
+        "npm pack should include .grok-plugin/plugin.json"
+      )
+      assert.ok(
+        packagedPaths.has(".grok-plugin/marketplace.json"),
+        "npm pack should include .grok-plugin/marketplace.json"
+      )
       assert.ok(
         packagedPaths.has(".agents/plugins/marketplace.json"),
         "npm pack should include .agents/plugins/marketplace.json"