chore: consolidate the skill packages under .agents/

The repository had two skill directories. `skills/` held the four written
for this project; `.agents/skills/` held the ones the agent context refers
to. Nothing said which an agent should read, and `.agents/ORCHESTRATOR.md`
only ever pointed at the second.

Move the first four into `.agents/skills/` so there is one location, and
add the vendored packages this chapter work used: `animation-vocabulary`
and `improve-animations` (emilkowalski/skills), `teach` (mattpocock/skills),
plus a local `translation` skill and `audit-translations.mjs` for the EN/PT
pairs.

`skills-lock.json` pins the vendored three by source and content hash, so a
later re-vendor is a diff rather than a guess. `.claude/skills/` is
symlinks into `.agents/skills/`, which is what makes them loadable here
without a second copy on disk.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.agents/skills/README.md

@@ -0,0 +1,18 @@
+# Reusable skills
+
+These project-local skills extract the design and implementation patterns used
+by AI For Dummies. They are intentionally small: copy a skill into an agent's
+skill directory, or give the `SKILL.md` path to an agent when building a new
+chapter.
+
+## Skills
+
+- [`editorial-playbook`](editorial-playbook/SKILL.md) — shape a content-led,
+  responsive, bilingual explainer with small interactive islands.
+- [`rules-case-study`](rules-case-study/SKILL.md) — turn repository rules,
+  skills, CLI checks, hooks, and review policy into a source-linked teaching
+  page.
+
+The reference files are deliberately disclosed beside each skill. The
+`evals/evals.json` files contain small prompts for checking that an agent
+reaches the right workflow.