docs(specs): add Kubernetes MCP design

[nickvigilante]

Summary

• Add the design spec for the Kubernetes MCP sub-project:
  read-only-by-default, gated-write access for Claude to the gandalf k3s cluster
  via self-hosted containers/kubernetes-mcp-server over Tailscale, with a
  least-privilege claude-mcp identity and Flux-managed RBAC.
• Docs only — no cluster or host changes. Review and implementation are deferred
  to Claude MCP access — Kubernetes: review spec, then implement #186 (drafted off the usual homelab machine).

Before merge

• No secrets in the diff — tokens referenced by Bitwarden item/field only.
• New persistent dirs — none (docs only).
• Host-level changes — none (docs only).
• SPOF impact — none (read-only external client; no new in-cluster service).
• DNS — no new hostname (uses gandalf's existing tailnet API endpoint).
• README walkthrough — n/a; design spec. Setup steps land with implementation (Claude MCP access — Kubernetes: review spec, then implement #186).

Test plan

• Review the spec against the real homelab machine (see Claude MCP access — Kubernetes: review spec, then implement #186 open questions: API endpoint, Tailscale ACL, token style, write scope, secrets exclusion, Bitwarden field).

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