feat: first-class skill packs on agents + ship gifsmith builtin

Lifts the 'an agent uses a SKILL.md pack' concept out of a host and into the
harness:
- run.Ports.SkillPacks (SkillPackActivator) — nil-safe port; the executor folds
  a loaded agent's pack catalog into the system prompt and adds a skill_use
  loader tool to the toolbox (uses the existing ra.SystemPrompt + toolbox seams)
- run.RunnableAgent.SkillPacks + persona.Agent.SkillPacks (+ skill_packs YAML,
  extends-inherit, ToRunnable) — the Agent noun is now pack-aware
- skillpack.Activator — the battery's default port impl (resolve names → packs →
  catalog + skill_use), with a per-run BundleStager factory the host plumbs;
  satisfies the port structurally (no import of run)
- agentbuiltins: ships gifsmith, a portable focused GIF/MP4 render agent that
  uses the gif pack — references tool/tier/pack NAMES only, no host coupling

A host now wires run.Ports.SkillPacks instead of carrying its own activation
glue. Tests: Activator resolution + gifsmith loads through persona→RunnableAgent.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

agentbuiltins/builtins_test.go

@@ -0,0 +1,42 @@
+package agentbuiltins_test
+
+import (
+	"context"
+	"slices"
+	"testing"
+
+	"gitea.stevedudenhoeffer.com/steve/executus/agentbuiltins"
+	"gitea.stevedudenhoeffer.com/steve/executus/persona"
+)
+
+// TestGifsmithLoads proves executus's shipped gifsmith manifest flows through
+// the persona loader and lowers into a RunnableAgent carrying the gif pack — the
+// path a host uses to dogfood it.
+func TestGifsmithLoads(t *testing.T) {
+	ctx := context.Background()
+	store := persona.NewMemory()
+	n, err := persona.LoadBuiltinAgents(ctx, store, agentbuiltins.FS(), nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n < 1 {
+		t.Fatalf("expected gifsmith seeded, got %d", n)
+	}
+	a, err := store.GetAgentByName(ctx, persona.BuiltinAgentOwnerID, "gifsmith")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(a.SkillPacks) != 1 || a.SkillPacks[0] != "gif" {
+		t.Errorf("skill_packs = %v", a.SkillPacks)
+	}
+	if a.ModelTier != "thinking" {
+		t.Errorf("model_tier = %q (want a portable tier name)", a.ModelTier)
+	}
+	if !slices.Contains(a.LowLevelTools, "code_exec") || !slices.Contains(a.LowLevelTools, "send_attachments") {
+		t.Errorf("low_level_tools missing render/deliver tools: %v", a.LowLevelTools)
+	}
+	// The pack must survive the lowering the executor consumes.
+	if ra := a.ToRunnable(); len(ra.SkillPacks) != 1 || ra.SkillPacks[0] != "gif" {
+		t.Errorf("RunnableAgent.SkillPacks = %v", ra.SkillPacks)
+	}
+}