Organize src/ into concern folders (http, auth, admin, plugin-host, ui); co-locate tests, move plugin-api barrel into plugin-host, sync docs + AGENTS layout

src/plugin-host/plugin-api.test.ts

@@ -0,0 +1,16 @@
+// The plugin author barrel: the stable surface a plugin imports. Guards that the value exports
+// stay present — removing one is a breaking contract change. The types resolve via typecheck (the
+// reference plugin imports them from here).
+import assert from "node:assert/strict";
+import test from "node:test";
+import * as api from "./plugin-api.ts";
+
+test("plugin-api re-exports the stable author value surface", () => {
+  for (const name of ["definePlugin", "can", "check", "GuardError", "requireSession", "parseListQuery", "readFormBody", "CSRF_FIELD", "tracedFetch", "Log", "safeUrl"]) {
+    assert.ok(name in api && api[name as keyof typeof api] !== undefined, `missing export: ${name}`);
+  }
+  assert.equal(typeof api.definePlugin, "function");
+  assert.equal(typeof api.tracedFetch, "function"); // the request-trace-aware fetch a plugin uses for upstream calls
+  assert.equal(api.safeUrl("javascript:alert(1)"), "#"); // the URL sanitiser for rendering untrusted hrefs
+  assert.equal(api.definePlugin({ apiVersion: "1.0.0" }).apiVersion, "1.0.0"); // identity helper works through the barrel
+});