Plugin SDK: guard package subpaths and fix Twitch setup export

* fix(plugins): add missing secret-input-schema build entry and Matrix runtime export

buildSecretInputSchema was not included in plugin-sdk-entrypoints.json,
so it was never emitted to dist/plugin-sdk/secret-input-schema.js. This
caused a ReferenceError during onboard when configuring channels that use
secret input schemas (matrix, feishu, mattermost, bluebubbles, nextcloud-talk, zalo).

Additionally, the Matrix extension's hand-written runtime-api barrel was
missing the re-export, unlike other extensions that use `export *` from
their plugin-sdk subpath.

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

* Plugin SDK: guard package subpaths and fix Twitch setup export

* Plugin SDK: fix import guardrail drift

---------

Co-authored-by: hxy91819 <masonxhuang@icloud.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/plugin-sdk/channel-runtime.ts

@@ -43,6 +43,7 @@ export * from "../channels/plugins/whatsapp-heartbeat.js";
 export * from "../infra/outbound/send-deps.js";
 export * from "../polls.js";
 export * from "../utils/message-channel.js";
+export * from "../whatsapp/normalize.js";
 export { createActionGate, jsonResult, readStringParam } from "../agents/tools/common.js";
 export * from "./channel-lifecycle.js";
 export * from "./directory-runtime.js";