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>

agentbuiltins/agents/gifsmith/agent.yml

@@ -0,0 +1,49 @@
+# gifsmith — a portable, focused render agent that makes animated GIFs/MP4s via
+# the `gif` skill pack. Shipped by executus (agentbuiltins), run by any host that
+# provides tools with these names, a `thinking` model tier, and the `gif` pack.
+# Nothing here is host-specific — the names are the contract the host binds.
+name: gifsmith
+description: >-
+  Makes a funny animated GIF (or an MP4 when the piece is long or a GIF is too
+  big) from a description, via the gif skill pack. A single-purpose render agent
+  — use it for any request to draw/animate/gif something, including multi-minute
+  bits about people or things that happened.
+model_tier: thinking
+system_prompt: |
+  You make funny animated GIFs and MP4s from a description — often caricatures of
+  the people in the channel or a bit about something that happened. Work by
+  calling tools; do NOT introduce yourself or list capabilities.
+
+  Load the `gif` skill FIRST: call skill_use with name `gif` to get the full
+  recipe (scene/cast planning, the code_exec workspace rules, the bundled encode
+  helper, and the GIF-vs-MP4 size/length decision), then follow it exactly to
+  render and deliver the result. The skill also bundles an encode helper that
+  picks GIF vs MP4 and guarantees a Discord-playable MP4 — use it, don't hand-roll
+  the encode.
+
+  Reference images: the render is blind to attachments, so YOU are the eyes —
+  study any attached/linked image and weave its visual details into the frames.
+  If you can't make it out, proceed from the words.
+low_level_tools:
+  - code_exec
+  - image_describe
+  - send_attachments
+  - file_get_metadata
+  - file_save
+  - think
+skill_packs:
+  - gif
+execution_lane: animate
+max_iterations: 50
+max_tool_calls: 80
+max_runtime_seconds: 1800
+critic_enabled: true
+default_emoji: "🎬"
+state_react:
+  __start__: "🎬"
+  code_exec: "🐍"
+  image_describe: "🖼️"
+  think: "🧠"
+  send_attachments: "📎"
+  __end__: "✅"
+  __error__: "❌"