Add backend resolver interface with file and env backends plus phase-3 stubs

Introduce the single Backend interface every credential source implements.
v0 ships two working backends: file (0600 KEY=VALUE env files, parsed in
pure Go, never sourced; looser permission masks refused before read) and
env (process-environment indirection). Bitwarden Secrets Manager and
HashiCorp Vault ship as explicit not-implemented stubs behind the same
interface so the phase-3 connectors are drop-ins. All failures are typed
ResolveErrors carrying only the ref, backend, and a fixed reason enum —
never material.

💘 Generated with Crush

Assisted-by: Crush:glm-5.2

internal/backend/env.go

@@ -0,0 +1,30 @@
+package backend
+
+import (
+	"context"
+	"os"
+)
+
+// Env resolves refs through process-environment indirection: ref.Source
+// is the variable name. This is the crush exposure-minimization pattern —
+// configs carry env var NAMES, never literals. The value is read on
+// demand and kept in memory only.
+type Env struct{}
+
+// NewEnv returns the env backend.
+func NewEnv() *Env { return &Env{} }
+
+// Name implements Backend.
+func (e *Env) Name() string { return "env" }
+
+// Resolve implements Backend.
+func (e *Env) Resolve(ctx context.Context, ref Ref) (string, error) {
+	v, ok := os.LookupEnv(ref.Source)
+	if !ok {
+		return "", Err(ref, e.Name(), ReasonMissingKey)
+	}
+	if v == "" {
+		return "", Err(ref, e.Name(), ReasonEmptyValue)
+	}
+	return v, nil
+}