fix: add main.js shim at plugin root for Obsidian loader

Obsidian's plugin loader expects main.js at the plugin root alongside
manifest.json — it does not resolve subdirectory paths in the 'main'
field.  Added a thin CommonJS shim that re-exports from dist/main.js.

- main.js: new entry shim (module.exports = require('./dist/main.js'))
- manifest.json: 'main' changed from 'dist/main.js' to 'main.js'
- install.sh: copy main.js shim to plugin folder, verify its presence
- README.md: add main.js to manual install instructions

main.js

@@ -0,0 +1,3 @@
+// Shim entry point — re-exports the compiled plugin from dist/
+// Obsidian expects main.js at the plugin root alongside manifest.json.
+module.exports = require('./dist/main.js');