docs: update INSTALLATION.md for consensus-mode + add operator runbook outline

[tridibnandy-sats]

Problem

docs/INSTALLATION.md lists NATS + Consul as prerequisites:

• NATS server running
• Consul server running

But the README clearly states the production path is the P2P ZAP consensus mode — 'No external dependencies.' A new operator following the installation guide would set up the deprecated legacy transport.

Additionally there is no operator runbook covering the day-to-day operations a production cluster needs:

• Key rotation procedure (add a share, reshare, retire old share)
• Health endpoint spec (what should a liveness probe hit, what should a readiness probe assert)
• Backup / restore from shares
• Node add / remove without resharing
• Deployment-validation smoke test (keygen → sign → verify)

Proposed changes

1. INSTALLATION.md rewrite — lead with consensus-mode deployment (docker compose, k8s StatefulSet with --peer flags). Move NATS/Consul to a 'Legacy transport' appendix with a deprecation banner.
2. docs/RUNBOOK.md (new) — operator checklist covering the items above. We'll land a stub with sections + TODOs; maintainers fill in the protocol-specific steps.
3. docs/HEALTH.md (new) — health endpoint spec: what each node exposes, what a K8s probe should check, recommended intervals.
4. scripts/smoke-test.sh — open a session, keygen a throwaway key, sign a nonce, verify the signature. Exit 0 if healthy, 1 otherwise. Callable from a K8s readiness init-container.

Context

This came up while reviewing the luxfi/mpc deployment shape used by downstream integrators running 5-node CGGMP21 clusters in Kubernetes. The gap between the README (consensus-first) and the installation guide (legacy-first) is the #1 onboarding friction.

Happy to contribute the PR — LMK if there's a style / location preference for the new docs.