Adds Obsidian Theme extension

[SECT19N]

Adds the Obsidian Theme extension, a dark theme inspired by the Notepad++ Obsidian Theme (specifically Nickc01's VS Code port) and the Zed Vercel Theme.

• Tested locally as a dev extension
• Extension ID and name do not contain "zed" or "extension"
• Submodule uses HTTPS URL
• Submodule path matches extension ID (extensions/obsidian-theme)
• Entry added to extensions.toml with correct version
• pnpm sort-extensions run
• License included in extension repository
• Theme JSON is inside themes/ directory
• Extension repository is publicly accessible

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• I've read CONTRIBUTING.md and followed the relevant guidance for adding or updating my extension.