feat(adapters): copy stdlib llm package (Client + Router) per ADR-004

Copy hyperguild/ingestion/internal/llm into internal/adapters/llm and own it.
Tapir owes that repo nothing at the dependency level — no module dep added.
Router gives the local-Primary -> BYO-Fallback path needed for ai_routing.feature.

Copied tests rewritten from testify to stdlib testing to keep go.mod
dependency-free (repo has zero deps; acceptance tests are stdlib too).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/adapters/llm/router.go

@@ -0,0 +1,29 @@
+package llm
+
+import (
+	"context"
+	"fmt"
+)
+
+// Router calls Primary first; on any error falls back to Fallback.
+// Fallback may be nil, in which case primary errors are returned directly.
+type Router struct {
+	Primary  *Client
+	Fallback *Client
+}
+
+// Complete routes through Primary then Fallback.
+func (r *Router) Complete(ctx context.Context, system, user string) (string, error) {
+	out, err := r.Primary.Complete(ctx, system, user)
+	if err == nil {
+		return out, nil
+	}
+	if r.Fallback == nil {
+		return "", fmt.Errorf("primary llm: %w", err)
+	}
+	out, err2 := r.Fallback.Complete(ctx, system, user)
+	if err2 != nil {
+		return "", fmt.Errorf("primary llm: %w; fallback llm: %v", err, err2)
+	}
+	return out, nil
+}