Pin the extension roadmap and downstream spike

Why: Downstream products need a published extension surface, contract checks, and a sample plugin that does not modify the base.
Evidence: scripts/lint-boundaries.sh printed boundaries ok. examples/downstream-rdm go test passed. Submodule pointers record the core, documents, backend, ui, and frontend commits for this roadmap.
Impact: make test runs the boundary lint first. make plugin-new scaffolds a declaration plugin. Version tags and remote downstream repositories are not created.
Refs: .task/feature-base-extension-roadmap.md
Co-authored-by: Cursor <cursoragent@cursor.com>

examples/plugins/decl-demo/channels/feishu.json

@@ -0,0 +1 @@
+{ "id": "feishu", "kind": "feishu", "title": "飞书" }

examples/plugins/decl-demo/knowledge/notes.json

@@ -0,0 +1 @@
+{ "id": "notes", "title": "笔记", "description": "数据集声明，检索引擎在基座" }

examples/plugins/decl-demo/page.mjs

@@ -0,0 +1,5 @@
+export function mount(el, bridge) {
+  el.textContent = '声明示例页面'
+  el.dataset.bridge = bridge ? 'ready' : ''
+}
+export function unmount() {}

examples/plugins/decl-demo/pages/home.json

@@ -0,0 +1 @@
+{ "id": "home", "title": "声明示例", "module": "./page.mjs" }

examples/plugins/decl-demo/plugin.json

@@ -0,0 +1,13 @@
+{
+  "id": "decl-demo",
+  "name": "声明示例",
+  "version": "0.1.0",
+  "apiVersion": "2.0-experimental",
+  "requires": { "network": ["api.example.com"] },
+  "provides": {
+    "skills": ["welcome"],
+    "pages": ["home"],
+    "channels": ["feishu"],
+    "knowledge": ["notes"]
+  }
+}

examples/plugins/decl-demo/skills/welcome.md

@@ -0,0 +1,3 @@
+# welcome
+
+声明式技能示例。描述任务步骤，不携带可执行代码。