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/ports.go

@@ -49,6 +49,22 @@ type Ports struct {
 	// are silently ignored (the run still proceeds, text-only). The bytes are
 	// never inlined into the model context — the LLM can't read raw audio/binary.
 	InputFiles InputFileStager
+	// SkillPacks activates a RunnableAgent.SkillPacks (SKILL.md subscriptions)
+	// for the run: it folds a catalog into the system prompt and adds a skill_use
+	// loader tool. nil = SkillPacks are inert. The executus/skillpack battery
+	// ships a default impl (skillpack.Activator).
+	SkillPacks SkillPackActivator
+}
+
+// SkillPackActivator resolves an agent's subscribed skill-pack names for a run
+// into system-prompt instructions (a catalog of what's available on demand) and
+// the tools that back them (a single skill_use loader). It receives the run +
+// subject ids so the impl can scope any per-run file staging. It returns "" +
+// nil when nothing resolves; activation errors are non-fatal to the run. Defined
+// here (the consumer) so the battery satisfies it structurally without importing
+// run — the same inversion as the other ports.
+type SkillPackActivator interface {
+	ActivateSkillPacks(ctx context.Context, names []string, runID, subjectID string) (instructions string, tools []llm.Tool, err error)
 }
 
 // InputFileStager persists a single non-image input attachment into a host file