chore: vendor documentation-writer + emil design/motion skills

[simpros]

Why the change

Vendor three agent skills (docs writing, design engineering, animation review) so agents in this repo can follow their clarify → outline → write, motion-craft, and flag-by-default review workflows.

Special things to note

• The three skills are vendored from github/awesome-copilot and emilkowalski/skills at the commits recorded in skills-lock.json, and only .agents/skills/**, skills-lock.json and AGENTS.md change.
• No code, docs/, or CI changes; no dependencies installed and no tests run.

Change outline

New vendored skills plus their pointer entries:

 .agents/skills/
+├── documentation-writer/SKILL.md  # Diátaxis page types + clarify → outline → write
+├── emil-design-eng/SKILL.md       # animation decisions, easing, feel, perf, a11y
+└── review-animations/             # flag-by-default motion review
+    ├── SKILL.md
+    └── STANDARDS.md               # the craft bar
 AGENTS.md                         # new "Vendored skills" subsection
 skills-lock.json                  # three new entries (source, skillPath, hash)