fix(bonjour): default LAN discovery on macOS only

Summary:
- add manifest-backed platform-specific default enablement for bundled plugins
- auto-start Bonjour LAN discovery on macOS hosts only
- keep Linux, Windows, and containerized Gateway deployments opt-in while preserving explicit enablement

Verification:
- pnpm test extensions/bonjour/src/advertiser.test.ts src/plugins/bundled-plugin-metadata.test.ts src/plugins/manifest-registry.test.ts src/plugins/channel-plugin-ids.test.ts
- pnpm exec oxfmt --check --threads=1 CHANGELOG.md docs/gateway/bonjour.md docs/gateway/configuration-reference.md docs/gateway/discovery.md docs/gateway/security/index.md docs/plugins/manifest.md extensions/bonjour/openclaw.plugin.json src/plugin-sdk/facade-activation-check.runtime.ts src/plugins/bundled-manifest-contract-plugins.ts src/plugins/bundled-plugin-metadata.test.ts src/plugins/channel-presence-policy.ts src/plugins/default-enablement.ts src/plugins/gateway-startup-plugin-ids.ts src/plugins/installed-plugin-index-record-builder.ts src/plugins/installed-plugin-index-store.ts src/plugins/installed-plugin-index-types.ts src/plugins/installed-plugin-index.ts src/plugins/loader.ts src/plugins/manifest-contract-eligibility.ts src/plugins/manifest-owner-policy.ts src/plugins/manifest-registry-installed.ts src/plugins/manifest-registry.test.ts src/plugins/manifest-registry.ts src/plugins/manifest.ts src/plugins/providers.ts
- git diff --check
- Testbox: pnpm check:changed via Blacksmith Testbox tbx_01kqqf3f8rbrt8afjtcg0ck7qs

Refs #74209

extensions/bonjour/openclaw.plugin.json

@@ -3,7 +3,7 @@
   "activation": {
     "onStartup": true
   },
-  "enabledByDefault": true,
+  "enabledByDefaultOnPlatforms": ["darwin"],
   "name": "Bonjour Gateway Discovery",
   "description": "Advertise the local OpenClaw gateway over Bonjour/mDNS.",
   "configSchema": {