feat: support explicit OpenClaw notifier destination routing

[topshark-jim]

Summary

• add optional channel and to passthrough fields to the OpenClaw notifier payload
• preserve existing behavior when destination overrides are omitted
• document fixed-destination routing and cover both default and explicit-routing cases in tests

Testing

• pnpm --filter @composio/ao-plugin-notifier-openclaw test
• pnpm --filter @composio/ao-integration-tests exec vitest run --config vitest.config.ts src/notifier-openclaw.integration.test.ts

Refs internal ticket TOP-104

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