feat(mcp): external registry + agent ctx + Streamable HTTP server

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/mcp/context.js

@@ -16,3 +16,25 @@ export function buildCtxFromEnv(env = process.env) {
     actor: { kind: 'user', id: null }
   };
 }
+
+/**
+ * Builds the tool ctx for an authenticated EXTERNAL agent. The Space is taken
+ * from the agent's own scope (never client-supplied) and `spaceScoped` is set
+ * so read() denies entities it can't prove are in-Space.
+ * @param {{id:string, capabilities?:object, scopes?:object}} agent
+ */
+export function buildCtxFromAgent(agent) {
+  const actor = {
+    kind: 'agent',
+    id: agent.id,
+    capabilities: agent.capabilities || {},
+    scopes: agent.scopes || {}
+  };
+  return {
+    agent: actor,
+    space_id: (agent.scopes && agent.scopes.space_id) || null,
+    view: null,
+    spaceScoped: true,
+    actor
+  };
+}