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/executor.go

@@ -275,6 +275,32 @@ func (e *Executor) Run(ctx context.Context, ra RunnableAgent, inv tool.Invocatio
 		postRun = st.PostRun
 	}
 
+	// Skill packs: resolve the agent's subscribed packs into a catalog (folded
+	// into the system prompt) + a skill_use loader tool added to the toolbox.
+	// nil-safe; activation failures are non-fatal — the run proceeds without
+	// packs rather than dying on a fetch/cache miss.
+	if len(ra.SkillPacks) > 0 && e.cfg.Ports.SkillPacks != nil {
+		instr, packTools, aerr := e.cfg.Ports.SkillPacks.ActivateSkillPacks(ctx, ra.SkillPacks, inv.RunID, ra.ID)
+		if aerr != nil {
+			slog.Warn("run: skill-pack activation failed; continuing without packs", "run_id", inv.RunID, "error", aerr)
+		} else {
+			for _, t := range packTools {
+				if err := toolbox.Add(t); err != nil {
+					res.Err = fmt.Errorf("add skill-pack tool: %w", err)
+					e.finishAudit(ctx, rec, "error", res, started, res.Err)
+					return res
+				}
+			}
+			if instr != "" {
+				if ra.SystemPrompt != "" {
+					ra.SystemPrompt += "\n\n" + instr
+				} else {
+					ra.SystemPrompt = instr
+				}
+			}
+		}
+	}
+
 	// Run context: detached from the caller's deadline so a lane/queue wait doesn't
 	// eat the run budget (mort's V10 lesson). Caller cancellation still propagates
 	// via MergeCancellation. Created BEFORE the step observer so the observer