Add Obsidian Claude skills external - #2

Summary

• Adds kepano/obsidian-skills as chezmoi externals
• Pins the skills archive to a1dc48e68138490d522c04cbf5822214c6eb1202
• Adds defuddle, json-canvas, obsidian-bases, obsidian-cli, and obsidian-markdown

Verification

• Validated TOML parsing
• Verified the pinned archive contains the expected skill files
• Verified chezmoi can materialize all five skill directories
• Ran git diff --check