Add Obsidian plugin

[wangjs-jacky]

Summary

• Add a zero-build Obsidian plugin for publishing the current Markdown note to OpenBird.
• Support authenticated publishing, temporary publishing, custom slugs, removal, saved per-note mappings, and local image upload/rewrite.
• Document manual Obsidian plugin installation in the root README and plugin README.

Test Plan

• cd obsidian-plugin && npm test
• cd obsidian-plugin && node --check main.js && node --check openbird-core.js
• node -e "const fs=require('fs'); JSON.parse(fs.readFileSync('obsidian-plugin/manifest.json','utf8')); JSON.parse(fs.readFileSync('obsidian-plugin/package.json','utf8')); console.log('json ok')"