feat: complete SHD MCP plugin workflows and widget

plugins/shd-mcp-plugin/skills/shd-discussions/references/discussion-contract.md

@@ -0,0 +1,14 @@
+# Discussion contract
+
+Use the narrowest tool for the request:
+
+- discovery: `shd_list_discussion_channels`,
+  `shd_list_discussion_topics`;
+- content: `shd_list_discussion_messages`, `shd_search_discussions`;
+- writes: `shd_add_discussion_message`, `shd_update_discussion_message`;
+- files: `shd_upload_discussion_attachment`.
+
+Resolve the canonical channel/topic/message identifiers before a write. Keep
+searches bounded, distinguish no results from denied scope, and retain the
+server's pagination or conflict fields. A message edit or attachment upload is
+a side effect: require explicit user intent and report the final server result.