feat(cluster): host-usable kubeconfig for local tenants (PR 2.5) + auto-derive lb-pool

[atbagan]

Problem

The local laptop path's headline step hung on macOS. butlerctl cluster kubeconfig <tenant> returned a kubeconfig pointing at the docker-bridge LoadBalancer IP (e.g. 172.18.255.201:6443), which is not routable from the macOS/Windows host, so kubectl get nodes hung. Two smaller copy-paste hazards in the documented create command: it required --provider (2 local PCs) and a machine-specific --lb-pool.

Changes

• Host-usable kubeconfig (PR 2.5). For local-provider tenants on macOS/Windows, cluster kubeconfig establishes a background kubectl port-forward to the tenant API server service (via the host-reachable management cluster) and rewrites the kubeconfig to https://127.0.0.1:<port>, preserving the original host as tls-server-name so the server cert (SANs include the bridge IP) still verifies. Reused across runs, detached so the follow-up kubectl works. On Linux the bridge IP is reachable directly, so the kubeconfig is emitted verbatim. Applies to stdout, -o, and --merge.
• Auto-derive lb-pool. cluster create derives the LoadBalancer pool from the kind docker network for local clusters when --lb-pool is omitted (mirrors bootstrap local), so the documented create command works without a machine-specific flag.
• TUI access hint in the cluster-detail view pointing at cluster kubeconfig.
• Unit test for the kubeconfig endpoint rewrite.

Verification (macOS, published artifacts, no --from-source)

Full literal walkthrough, copy-paste as documented:

butlerctl cluster create toy --provider butler-local-provider --workers 1   # auto-derives lb-pool -> Ready
butlerctl cluster kubeconfig toy > toy.yaml                                  # 127.0.0.1 + tls-server-name, tunnel auto-started
kubectl --kubeconfig toy.yaml get nodes                                      # Ready node, NO manual port-forward

All three kubeconfig output paths (stdout / -o / --merge) verified. go vet clean, tests pass.

Pairs with the docs fix in butlerdotdev/butler (local.mdx).