feat(console): serve web channels to allowlisted targets

[traviswu-bigstack]

/kind feature

What

The node-side half of the web console (#9): a web tunnel target, resolved
against an allowlist the node owns.

• internal/console/web.go — WebAllowlist, one symbolic name to exactly one
  host:port, loaded from /etc/cube-advisor-agent/web-targets.json
• serveConsole gains a TargetWeb arm beside the existing SSH one
• -web-targets configures the file; absent means the node exposes no web
  target and still serves SSH, malformed stops the agent at startup

Why

ADR 0002 says the node decides what it exposes. A web channel is the same
bargain as the SSH one: the SaaS names a target symbolically and the node maps
it, so a compromised SaaS cannot reach an address the node never offered.

Notes for reviewers

• One name, one address — not a set. Target carries a name and nothing
  else, so a name meaning several addresses could not say which it wanted. An
  app and its identity provider are two names; grouping is the SaaS's job.
• JSON, not YAML. This runs on every customer node and does not take a
  dependency to read four lines of config.
• Every refusal returns the generic RefusedReason; the distinguishing detail
  is logged locally and never sent, so a probing SaaS cannot map the cluster by
  watching which refusals differ.
• serveSSHConsole is the pre-existing SSH logic extracted unchanged.
• Verified on sky141: a console and 3 web targets for ky3haclust01/sky141,
  and CMP reached through it end to end.

Part of #9 — not Fixes, because #9's criteria include nested WebSocket,
which is a later slice.

🤖 Generated with Claude Code

https://claude.ai/code/session_01PZ5umjjCedZwWtbAbiMjfj