Prepare for open source: Apache-2.0 license, contributing guide, self-hosting guide, fix Close()

- LICENSE (Apache-2.0), NOTICE, CONTRIBUTING.md
- docs/self-hosting.md and README sections for hosting and contributing
- Close() no longer crashes: it only disconnects and leaves the slash commands registered, so they keep their IDs across restarts
- Tests use a neutral server ID

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

internal/beta/beta_test.go

@@ -8,7 +8,7 @@ import (
 )
 
 const (
-	betaGuild  = "1170934850157547600"
+	betaGuild  = "700000000000000001"
 	otherGuild = "999000000000000001"
 	testerRol  = "555000000000000001"
 	testerID   = "777000000000000001"