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>

skillpack/activator.go

@@ -0,0 +1,58 @@
+package skillpack
+
+import (
+	"context"
+	"errors"
+
+	"gitea.stevedudenhoeffer.com/steve/majordomo/llm"
+)
+
+// Activator adapts the battery to executus/run's SkillPackActivator port: given
+// an agent's subscribed pack names, it resolves them to their pinned packs and
+// returns the catalog instructions + the skill_use tool the run injects. It
+// satisfies run.SkillPackActivator structurally — no import of run — so the
+// battery stays run-agnostic (the same inversion as the other batteries).
+//
+// StagerFor, when set, builds the per-run BundleStager (a host plumbs bundled
+// files into its own run-scoped storage from the run + subject ids); nil means
+// skill_use lists a pack's bundled filenames without staging them.
+type Activator struct {
+	Cache     PackCache
+	Subs      Store
+	StagerFor func(runID, subjectID string) BundleStager
+}
+
+// ActivateSkillPacks implements run.SkillPackActivator. Unknown or disabled pack
+// names are skipped; it returns "" + nil when nothing resolves.
+func (a *Activator) ActivateSkillPacks(ctx context.Context, names []string, runID, subjectID string) (string, []llm.Tool, error) {
+	if a == nil || a.Subs == nil || a.Cache == nil || len(names) == 0 {
+		return "", nil, nil
+	}
+	chosen := make([]Subscription, 0, len(names))
+	for _, n := range names {
+		sub, err := a.Subs.GetByName(ctx, n)
+		if errors.Is(err, ErrNotFound) {
+			continue
+		}
+		if err != nil {
+			return "", nil, err
+		}
+		if !sub.Enabled {
+			continue
+		}
+		chosen = append(chosen, *sub)
+	}
+	packs, err := Resolve(ctx, a.Cache, chosen)
+	if err != nil {
+		return "", nil, err
+	}
+	var stager BundleStager
+	if a.StagerFor != nil {
+		stager = a.StagerFor(runID, subjectID)
+	}
+	sk := Activate(packs, stager)
+	if sk == nil {
+		return "", nil, nil
+	}
+	return sk.Instructions(), sk.Tools().Tools(), nil
+}

skillpack/activator_test.go

@@ -0,0 +1,50 @@
+package skillpack
+
+import (
+	"context"
+	"testing"
+)
+
+func TestActivator(t *testing.T) {
+	ctx := context.Background()
+	src := &fakeSource{tree: packTree("alpha", "do alpha things"), ref: "r1"}
+	y := newTestSyncer(src)
+	if _, err := y.Subscribe(ctx, src, "main", "steve"); err != nil {
+		t.Fatal(err)
+	}
+
+	staged := 0
+	act := &Activator{
+		Cache: y.Cache, Subs: y.Subs,
+		StagerFor: func(runID, subjectID string) BundleStager {
+			return func(context.Context, *Pack) (string, error) { staged++; return "", nil }
+		},
+	}
+
+	instr, tools, err := act.ActivateSkillPacks(ctx, []string{"alpha"}, "run1", "agent1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if instr == "" {
+		t.Error("expected catalog instructions")
+	}
+	found := false
+	for _, tl := range tools {
+		if tl.Name == "skill_use" {
+			found = true
+		}
+	}
+	if !found {
+		t.Errorf("expected a skill_use tool, got %d tools", len(tools))
+	}
+
+	// unknown name → nothing resolves (no error, no tools).
+	if in, tl, err := act.ActivateSkillPacks(ctx, []string{"nope"}, "r", "a"); err != nil || in != "" || tl != nil {
+		t.Fatalf("unknown pack should resolve to nothing: in=%q tools=%v err=%v", in, tl, err)
+	}
+
+	// nil-safe: a zero Activator (or empty names) is inert.
+	if in, tl, err := (&Activator{}).ActivateSkillPacks(ctx, []string{"alpha"}, "r", "a"); err != nil || in != "" || tl != nil {
+		t.Fatalf("zero Activator should be inert: %q %v %v", in, tl, err)
+	}
+}