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>

run/agent.go

@@ -44,6 +44,11 @@ type RunnableAgent struct {
 	LowLevelTools   []string
 	SkillPalette    []string
 	SubAgentPalette []string
+	// SkillPacks names SKILL.md skill-pack subscriptions activated for the run
+	// via Ports.SkillPacks: each pack's name+description joins a catalog folded
+	// into the system prompt, and a skill_use tool loads a pack's body on demand
+	// (progressive disclosure). nil Ports.SkillPacks => inert.
+	SkillPacks []string
 
 	// Phases optionally model a multi-step pipeline (each phase its own prompt
 	// + tier + tools). An empty slice is a single-phase run — the common case.