Add the provider Checker contract and the kubernetes and GCP checks

Checker is an optional interface next to Provider: Check returns one
verdict per target and never fails as a whole. The kubernetes provider
lists at most one managed Pod cluster-wide (the verb and scope GC relies
on); the GCP provider fetches a single page of at most one managed
instance through a new Probe seam method, since the SDK treats
MaxResults as a page size and the existing AggregatedList drains every
page.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/provider/check.go

@@ -0,0 +1,46 @@
+package provider
+
+import (
+	"context"
+	"time"
+)
+
+// Reasons shared by every Checker, so a listing reads the same across
+// providers. Providers add their own for failures only they can tell
+// apart (KVM's SSHHandshakeFailed or PoolNotActive, say).
+const (
+	ReasonReachable    = "Reachable"
+	ReasonTimeout      = "Timeout"
+	ReasonUnreachable  = "Unreachable"
+	ReasonUnauthorized = "Unauthorized"
+	ReasonForbidden    = "Forbidden"
+	ReasonAPIError     = "APIError"
+)
+
+// TargetStatus is one target's verdict from a Checker.
+type TargetStatus struct {
+	// Name identifies the target within its provider: a KVM host name,
+	// "apiserver", a GCP project.
+	Name string
+	// Address is what the check connected to, for humans.
+	Address string
+	Ready   bool
+	// Reason is ReasonReachable when Ready, otherwise why not.
+	Reason string
+	// Message is human-readable detail, usually the error.
+	Message string
+	Latency time.Duration
+	// Details are provider-specific facts gathered by the check.
+	Details map[string]string
+}
+
+// Checker is the optional interface of a provider that can verify its
+// backend is reachable and usable without touching any instance. The
+// provider health monitor type-asserts for it on the unwrapped providers,
+// so it stays out of the Provider contract.
+type Checker interface {
+	// Check returns one verdict per target, in a stable order. It never
+	// fails as a whole — a failure is a verdict — and must return promptly
+	// once ctx ends.
+	Check(ctx context.Context) []TargetStatus
+}