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/agent.go

@@ -86,6 +86,9 @@ type Agent struct {
 	SkillPalette    []string // skill IDs/names
 	SubAgentPalette []string // agent IDs/names
 	LowLevelTools   []string // skilltools registry names
+	// SkillPacks names SKILL.md skill-pack subscriptions activated for a run via
+	// run.Ports.SkillPacks (catalog folded into the prompt + a skill_use loader).
+	SkillPacks []string
 
 	// Personalization (Phase 5 reads these). Each layer name maps to
 	// a registered PersonalizationProvider that returns text appended