feat(plugins): plugin hook wiring + comprehensive test suite + welcome-banner example (#3045)

Follow-up to the plugins framework (#3041): wires the plugin hooks end-to-end and adds full test coverage.

- Wires `onRequest` / `onResponse` / `onError` hooks into the chat pipeline (`chatCore` now imports from the unified `hooks` registry).
- Loads active plugins on server startup (`pluginManager.loadAll()` in `server-init`) so they survive restarts.
- Ships a `welcome-banner` example plugin (`examples/plugins/`) + test fixture.
- Adds a comprehensive plugin test suite (manifest, db, config, hooks, manager lifecycle, loader IPC, permissions, scanner, welcome-banner e2e).

Integration fixes applied during review:
- `manager.install` now removes an orphaned `destDir` (DB row gone but files left on disk) before the atomic rename, guarded by path containment — it previously failed with `ENOTEMPTY` (a regression surfaced by the new lifecycle test).
- `plugins-db` / `plugins-manager-lifecycle` tests now initialize the DB via the real migration `076` (`getDbInstance`) rather than relying on ambient state, so a missing/renumbered migration fails loudly instead of being masked.

348/348 plugin tests pass; typecheck / cycles clean.

Co-authored-by: oyi77 <14921983+oyi77@users.noreply.github.com>

examples/plugins/welcome-banner/index.mjs

@@ -0,0 +1,36 @@
+/**
+ * Welcome Banner Plugin — PoC demonstrating the OmniRoute plugin system.
+ *
+ * Adds a banner message to request metadata on every request.
+ * Logs a delivery confirmation on every response.
+ *
+ * @module welcome-banner
+ */
+
+/**
+ * onRequest hook — injects banner text into request metadata.
+ *
+ * @param {object} ctx - Plugin context
+ * @param {object} [ctx.config] - Plugin configuration
+ * @param {object} [ctx.metadata] - Request metadata (mutable)
+ */
+export function onRequest(ctx) {
+  const config = ctx?.config || {};
+  const enabled = config.enabled !== false; // default true
+  if (!enabled) return;
+
+  const bannerText = config.bannerText || "Welcome to OmniRoute!";
+  if (ctx.metadata) {
+    ctx.metadata.banner = bannerText;
+  }
+}
+
+/**
+ * onResponse hook — fire-and-forget banner delivery log.
+ *
+ * @param {object} ctx - Plugin context
+ * @param {object} response - Upstream response
+ */
+export function onResponse() {
+  // No-op — banner is request-side only
+}

examples/plugins/welcome-banner/plugin.json

@@ -0,0 +1,29 @@
+{
+  "name": "welcome-banner",
+  "version": "1.0.0",
+  "description": "Adds a welcome banner to API responses",
+  "author": "OmniRoute",
+  "license": "MIT",
+  "main": "index.mjs",
+  "source": "local",
+  "tags": ["demo", "banner"],
+  "hooks": {
+    "onRequest": true,
+    "onResponse": true,
+    "onError": false
+  },
+  "permissions": [],
+  "enabledByDefault": true,
+  "configSchema": {
+    "bannerText": {
+      "type": "string",
+      "default": "Welcome to OmniRoute!",
+      "description": "Banner message to display"
+    },
+    "enabled": {
+      "type": "boolean",
+      "default": true,
+      "description": "Enable or disable the banner"
+    }
+  }
+}