Fork Notesium source and restructure into Go package layout

Initial fork of github.com/alonswartz/notesium into librenotes:
- Source moved to internal/notesium/ (package notesium)
- Thin entry point at cmd/librenotes/main.go
- Module renamed to git.librete.ch/public/librenotes
- main() exposed as notesium.Run()
- LICENSE preserved (MIT), NOTICE added with attribution
- Web assets and completion.bash co-located with embedding code
  to satisfy go:embed path constraints

Closes #3, #34, #35.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/notesium/web/index.html

@@ -0,0 +1,10 @@
+<!DOCTYPE html>
+<html>
+<head>
+  <title>Redirecting...</title>
+  <meta http-equiv="refresh" content="0;url=/app/">
+</head>
+<body>
+  <p>If you are not redirected, <a href="/app/">click here</a>.</p>
+</body>
+</html>