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.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 }