Add Claude Code plugin manifest; update for Obsidian v1.12 public release

- Add plugin.json for Claude Code plugin system compatibility
- Remove Catalyst License and Early Access/Insider requirements — Obsidian v1.12
  is now available to all users
- Update README: Claude Code plugin install instructions with /plugin command,
  manual ZIP install option, and corrected prerequisites
- Fix Windows platform note (remove insider Discord channel reference)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plugin.json

@@ -0,0 +1,8 @@
+{
+  "name": "obsidian-cli",
+  "version": "1.0.0",
+  "description": "Interact with Obsidian vaults using the official Obsidian CLI (v1.12+). Read, create, search, and manage notes, daily notes, properties, tags, tasks, bookmarks, templates, sync, plugins, and links — all from the terminal.",
+  "author": "pablo-mano",
+  "repository": "https://github.com/pablo-mano/Obsidian-CLI-skill",
+  "skills": ["."]
+}