docs: add deployment and opencode examples

[typelicious]

What changed

• added a fuller OpenClaw example for FoundryGate as the primary chat gateway
• added a generic multi-provider FoundryGate stack example for OpenClaw, n8n, CLI, local workers, and image providers
• added an opencode custom-provider example that points at FoundryGate
• linked the new examples from the integrations, onboarding, and root README docs

Why

This gives operators a copy-paste starting point for a real multi-provider deployment without baking private environment details into the public repo.

How verified

• python3 -m json.tool docs/examples/opencode-foundrygate.json
• git diff --check