Skills standard compliance

Implement Agent Skills standard (https://agentskills.io/specification):
- Validate name (must match parent dir, lowercase, max 64 chars)
- Validate description (required, max 1024 chars)
- Warn on unknown frontmatter fields
- Warn on name collisions (keep first)
- Change prompt format to XML structure
- Remove {baseDir} placeholder (use relative paths)
- Add tests and update documentation

fixes #231

packages/coding-agent/test/fixtures/skills/unknown-field/SKILL.md

@@ -0,0 +1,10 @@
+---
+name: unknown-field
+description: A skill with an unknown frontmatter field.
+author: someone
+version: 1.0
+---
+
+# Unknown Field
+
+This skill has non-standard frontmatter fields.