docs(skills): add a spec review skill and the skills index

skills/README.md

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+# Skills for writing sanderling specs
+
+Agent skills for adopting sanderling: getting it running against your app, writing
+property specifications, reviewing them for the failure modes that make a spec
+look like it works when it does not, and reading a run honestly.
+
+Copy the ones you want into your agent's skills directory (`.claude/skills/` for
+Claude Code) or point your agent at this directory directly.
+
+Start with `sanderling-setup`, then `sanderling-spec-authoring`. Run
+`sanderling-spec-review` over anything before you trust it.
+
+The reasoning behind the rules these encode is in
+[docs/development/design-principles.md](../docs/development/design-principles.md),
+section 8 in particular.