Add the provider contract, error taxonomy, naming, and config (Step 2)

The Provider interface (Create/Get/Delete/ListByTag), Instance, and
CreateRequest that every cloud backend implements — kept independent of
api/v1alpha1 so this package has no CRD-type coupling.

Error taxonomy (ErrNotFound/ErrQuotaExceeded/ErrTransient/ErrPermanent)
wrapped via a multi-error Unwrap() []error, so errors.Is and errors.As
both work off the same value: the reconciler branches on classification,
logs keep the underlying SDK error. Unclassified errors default to
ErrTransient — retrying is always safer than latching Failed.

Deterministic instance naming (SHA-256 -> base32 -> 16 chars, 22 total
with the "proxy-" prefix) satisfying GCP's RFC1035 name rules with
headroom, and idempotency-tested across 10k UIDs with zero collisions.

--providers-config YAML parsing (config.go) with fail-fast validation:
unknown type, duplicate name, missing gcp.project, mismatched
type/config-block, and strict-mode rejection of unknown keys.

internal/provider/registry/registry.go takes its type->constructor map
as a parameter rather than hardcoding it, so the package has zero import
on internal/provider/mock or internal/provider/gcp (neither exists yet —
mock is Step 3, gcp is Step 8) and compiles today. Explicit wiring moves
to the composition root in cmd/main.go (Step 10).

Deferred internal/provider/metrics.go (the WithMetrics decorator) to
Step 9, where the Prometheus vectors it needs actually get built —
nothing in this step depends on it.

internal/provider at 96.2% coverage, internal/provider/registry at 100%.
make test green.

Co-Authored-By: Claude <noreply@anthropic.com>

go.mod

@@ -8,6 +8,7 @@ require (
 	k8s.io/apimachinery v0.36.0
 	k8s.io/client-go v0.36.0
 	sigs.k8s.io/controller-runtime v0.24.1
+	sigs.k8s.io/yaml v1.6.0
 )
 
 require (
@@ -96,5 +97,4 @@ require (
 	sigs.k8s.io/json v0.0.0-20250730193827-2d320260d730 // indirect
 	sigs.k8s.io/randfill v1.0.0 // indirect
 	sigs.k8s.io/structured-merge-diff/v6 v6.3.2 // indirect
-	sigs.k8s.io/yaml v1.6.0 // indirect
 )