chore: bump version to 0.4.0

fix: compatibility with OpenClaw >=2026.3.24

- Replace openclaw/plugin-sdk/synology-chat imports (removed in v2026.3.24):
  - buildChannelConfigSchema, DEFAULT_ACCOUNT_ID, setAccountEnabledInConfigSection -> plugin-sdk/core
  - registerPluginHttpRoute -> plugin-sdk/webhook-ingress
  - OpenClawPluginApi type -> plugin-sdk/core
- Rewrite entry point to use defineChannelPluginEntry (new API)
- Add kind: channel to openclaw.plugin.json
- Bump peerDependency to >=2026.3.24
- Update README with correct installation and config instructions
- Document plugins.allow gotcha (blocks bundled plugins like Telegram)

0.2.1