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>

persona/runnable.go

@@ -18,6 +18,7 @@ func (a *Agent) ToRunnable() run.RunnableAgent {
 		LowLevelTools:   a.LowLevelTools,
 		SkillPalette:    a.SkillPalette,
 		SubAgentPalette: a.SubAgentPalette,
+		SkillPacks:      a.SkillPacks,
 		Critic: run.CriticConfig{
 			Enabled:            a.CriticEnabled,
 			BackstopMultiplier: a.CriticBackstopMultiplier,