Accept story 40: MCP Server Obeys STORYKIT_PORT

Agent worktrees now get a .mcp.json written with the correct port from
the running server. AgentPool receives the port at construction and
passes it through to create_worktree, which writes .mcp.json on both
new creation and reuse.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/http/context.rs

@@ -23,7 +23,7 @@ impl AppContext {
             state: Arc::new(state),
             store: Arc::new(JsonFileStore::new(store_path).unwrap()),
             workflow: Arc::new(std::sync::Mutex::new(WorkflowState::default())),
-            agents: Arc::new(AgentPool::new()),
+            agents: Arc::new(AgentPool::new(3001)),
         }
     }
 }