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/builtin_loader.go

@@ -291,6 +291,9 @@ func resolveExtends(child, parent *Agent) {
 	if child.LowLevelTools == nil {
 		child.LowLevelTools = parent.LowLevelTools
 	}
+	if child.SkillPacks == nil {
+		child.SkillPacks = parent.SkillPacks
+	}
 	if child.PersonalizationSources == nil {
 		child.PersonalizationSources = parent.PersonalizationSources
 	}
@@ -456,6 +459,7 @@ type builtinAgentManifest struct {
 	SkillPalette    []string `yaml:"skill_palette"`
 	SubAgentPalette []string `yaml:"sub_agent_palette"`
 	LowLevelTools   []string `yaml:"low_level_tools"`
+	SkillPacks      []string `yaml:"skill_packs"`
 
 	PersonalizationSources []string `yaml:"personalization_sources"`
 
@@ -562,6 +566,7 @@ func decodeAgentManifest(data []byte) (*Agent, error) {
 		SkillPalette:             m.SkillPalette,
 		SubAgentPalette:          m.SubAgentPalette,
 		LowLevelTools:            m.LowLevelTools,
+		SkillPacks:               m.SkillPacks,
 		PersonalizationSources:   m.PersonalizationSources,
 		Schedule:                 strings.TrimSpace(m.Schedule),
 		WebhookIPAllowlist:       allowlist,