feat: multi-system cluster install mode (standalone/seed/member)

[growlf]

Design from F/multi-system-install-cluster-mode branch, preserved before that branch closes.

Summary

Add three installation modes to install.sh: Start Cluster, Join Cluster, and Standalone. Re-runnable so any node can change its role later.

Core concept: CLUSTER_ROLE in .env

CLUSTER_ROLE=standalone|seed|member
CLUSTER_NAME=ai-cluster
CLUSTER_ID= # generated once per seed

• standalone: current behavior (default)
• seed: first node — originates cluster, hosts node registry
• member: joins existing cluster

Discovery

Seed nodes pull a marker model ai-stack-cluster:latest (~1MB). Joiners scan LAN port 11434 for this model — no extra infrastructure.

Files to change

• install.sh — mode selection menu, scan/join/seed-init functions
• .env.example — add CLUSTER_ROLE, CLUSTER_NAME, CLUSTER_ID (commented)
• scripts/change-cluster-role.sh (new)
• scripts/discover-network.sh — export scan_lan_ollama() as library

Notes

• Full design in PLANS.md on branch F/multi-system-install-cluster-mode
• Depends on PRs fix(router): add capability registry, fix tools routing, rename to PEP8 #11, docs: revamp README, add comprehensive docs/ structure #12, feat(hardware): make docker-compose hardware-agnostic #13 merging first
• Related stretch goal: NetBird VPN for cross-network clusters