feat: make local kubeconfig auto-discovery explicit, countable, and opt-out

[jabbott-iii]

What does this PR do?

Discovery contract: explicit local kubeconfig telemetry

Added localKubeconfigAutoDiscovery to snapshot metadata and graph payload.
Contract fields:

• enabled: whether local kubeconfig auto-discovery is allowed
• ran: whether local kubeconfig discovery logic actually executed
• discoveredContexts: contexts parsed from local kubeconfig sources
• connectedContexts: contexts that produced connected local k8s discovery
  Contract is always present in graph snapshot shape.

Resolver refactor: local kubeconfig discovery is now explicit

Introduced a dedicated kubeconfig resolver path (ResolveKubeConfig) that:

• tries in-cluster config first
• conditionally runs local kubeconfig discovery (KUBECONFIG path-list and ~/.kube/config)
  Local context counting is derived from parsed kubeconfig data, without cluster calls.

Opt-out controls: CLI + environment

Added --discover-local-kubeconfig to both serve and start (default true).
Added INFRACANVAS_DISCOVER_LOCAL_KUBECONFIG override.
Wired this setting through both:
Kubernetes discovery initialization
Kubernetes action executor initialization
Explicit uploaded Cluster connections (kubeconfig via UI) remain unaffected.

UI: one-time local auto-discovery notice

Added a dismissible first-load notice when local kubeconfig contexts were auto-discovered and connected.
Notice is shown only for local host session and only when ran && discoveredContexts > 0 && connectedContexts > 0.
Dismissal is persisted in localStorage to avoid repeated display across refresh/reconnect.
No notice for disabled discovery, in-cluster-only config, zero contexts, or explicit uploaded Cluster sessions.
Docs/security updates

Updated README and SECURITY docs with:

1. explicit local kubeconfig auto-discovery behavior
2. opt-out flag/env
3. example usage
   Updated command help environment variable sections accordingly.

Type of change

• Bug fix
• New feature
• Refactor
• Documentation
• CI / tooling

Testing

• make test passes
• make lint passes
• Manually tested against a real VM (for agent changes)
• Frontend builds and UI looks correct (npm run build)

Related issues

closes #15

[rishibaghel25]

LGTM