Split agent-skills-spec into separate authoring and client integration guides

Reorganize the spec documentation:
- agent-skills-spec.md now serves as an index linking to the guides
- skill-authoring.md covers skill creation and SKILL.md format
- skill-client-integration.md provides guidance for Skill Client implementors

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

spec/skill-authoring.md

@@ -0,0 +1,47 @@
+# Skill Authoring Guide
+
+This guide describes how to create skills that agents can discover and use.
+
+## Skill Folder Layout
+
+A minimal skill folder looks like this:
+
+```
+my-skill/
+  - SKILL.md
+```
+
+More complex skills can add additional directories and files as needed.
+
+
+## The SKILL.md File
+
+The skill's "entrypoint" is the `SKILL.md` file. It is the only file required to exist. The file must start with a YAML frontmatter followed by regular Markdown.
+
+### YAML Frontmatter
+
+The YAML frontmatter has 2 required properties:
+
+- `name`
+    - The name of the skill in hyphen-case
+    - Restricted to lowercase Unicode alphanumeric + hyphen
+    - Must match the name of the directory containing the SKILL.md
+- `description`
+    - Description of what the skill does and when Claude should use it
+
+There are 3 optional properties:
+
+- `license`
+    - The license applied to the skill
+    - We recommend keeping it short (either the name of a license or the name of a bundled license file)
+- `allowed-tools`
+    - A list of tools that are pre-approved to run
+    - Currently only supported in Claude Code
+- `metadata`
+    - A map from string keys to string values
+    - Clients can use this to store additional properties not defined by the Agent Skills Spec
+    - We recommend making your key names reasonably unique to avoid accidental conflicts
+
+### Markdown Body
+
+The Markdown body has no restrictions on it.