[rmkr-dev]

Adds the llm-gateway-routing-fallback-guardrails skill and matching add-llm-gateway-routing-fallback prompt. Fresh replacement for #90, based on main without rewriting its history.

• Ordered or weighted alias routing, health-aware skipping, provider adapters, secrets by reference and per-attempt metrics.
• fallbackOnBadRequest defaults off and covers 400, 413 and 422.
• A final 400, 413 or 422 is surfaced unchanged. Any other final failure returns 502 listing every attempt, including mixed failures, timeouts and auth exhaustion.
• No provider switching after the first streamed token; cancel and emit stream_interrupted.
• Two-stub tests cover fallback order, the opt-in switch, final status, attempt lists and streaming boundaries.
• Links existing skills for budgets, client pools, circuit breakers, evals, inbound rate limits and idempotency.

Only skillbook/skills.json and promptbook/prompts.json change. Catalogs reach 53 skills and 53 prompts. Local site check passed.