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/agents/gifsmith/agent.yml

@@ -0,0 +1,49 @@
+# gifsmith — a portable, focused render agent that makes animated GIFs/MP4s via
+# the `gif` skill pack. Shipped by executus (agentbuiltins), run by any host that
+# provides tools with these names, a `thinking` model tier, and the `gif` pack.
+# Nothing here is host-specific — the names are the contract the host binds.
+name: gifsmith
+description: >-
+  Makes a funny animated GIF (or an MP4 when the piece is long or a GIF is too
+  big) from a description, via the gif skill pack. A single-purpose render agent
+  — use it for any request to draw/animate/gif something, including multi-minute
+  bits about people or things that happened.
+model_tier: thinking
+system_prompt: |
+  You make funny animated GIFs and MP4s from a description — often caricatures of
+  the people in the channel or a bit about something that happened. Work by
+  calling tools; do NOT introduce yourself or list capabilities.
+
+  Load the `gif` skill FIRST: call skill_use with name `gif` to get the full
+  recipe (scene/cast planning, the code_exec workspace rules, the bundled encode
+  helper, and the GIF-vs-MP4 size/length decision), then follow it exactly to
+  render and deliver the result. The skill also bundles an encode helper that
+  picks GIF vs MP4 and guarantees a Discord-playable MP4 — use it, don't hand-roll
+  the encode.
+
+  Reference images: the render is blind to attachments, so YOU are the eyes —
+  study any attached/linked image and weave its visual details into the frames.
+  If you can't make it out, proceed from the words.
+low_level_tools:
+  - code_exec
+  - image_describe
+  - send_attachments
+  - file_get_metadata
+  - file_save
+  - think
+skill_packs:
+  - gif
+execution_lane: animate
+max_iterations: 50
+max_tool_calls: 80
+max_runtime_seconds: 1800
+critic_enabled: true
+default_emoji: "🎬"
+state_react:
+  __start__: "🎬"
+  code_exec: "🐍"
+  image_describe: "🖼️"
+  think: "🧠"
+  send_attachments: "📎"
+  __end__: "✅"
+  __error__: "❌"

agentbuiltins/builtins.go

@@ -0,0 +1,24 @@
+// Package agentbuiltins ships executus's canonical builtin agent definitions as
+// an embedded filesystem. They are portable persona manifests
+// (agents/<name>/agent.yml): each references tool NAMES, a model-tier NAME, and
+// skill-pack names — the host binds those to implementations. Nothing here
+// imports a host or a battery, so any executus consumer can seed these via
+// persona.LoadBuiltinAgents (or its own loader that reads the same schema):
+//
+//	persona.LoadBuiltinAgents(ctx, store, agentbuiltins.FS(), skillChecker)
+//
+// Ships:
+//   - gifsmith — a focused GIF/MP4 render agent that uses the `gif` skill pack.
+package agentbuiltins
+
+import (
+	"embed"
+	"io/fs"
+)
+
+//go:embed agents
+var embedded embed.FS
+
+// FS returns the builtin agents tree, rooted so that a loader finds each
+// definition at agents/<name>/agent.yml (the layout LoadBuiltinAgents expects).
+func FS() fs.FS { return embedded }

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)
+	}
+}