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/bot/beta_sync_test.go

@@ -20,7 +20,7 @@ import (
 
 const (
 	appID      = "100"
-	betaServer = "1170934850157547600"
+	betaServer = "700000000000000001"
 	optedIn    = "222"
 	leftServer = "333"
 )
@@ -219,3 +219,22 @@ func TestBetaCleanupIsRetriedAndGivesUpWhenTheBotIsGone(t *testing.T) {
 		t.Error("a server the bot can't reach any more is forgotten")
 	}
 }
+
+// Close used to loop over a list that started with empty entries and read
+// their IDs, so shutting down crashed. It must just disconnect, and must not
+// touch the registered commands.
+func TestCloseDoesNotTouchCommands(t *testing.T) {
+	d := &fakeDiscord{cmds: map[string][]*discordgo.ApplicationCommand{
+		"1": {{ID: "7", Name: "ping"}},
+	}, failures: map[string]int{}}
+	b := newTestBot(t, d)
+	b.GuildID = "1"
+	b.Commands = []*discordgo.ApplicationCommand{{Name: "ping"}}
+
+	if err := b.Close(); err != nil {
+		t.Fatalf("Close on a bot that never connected: %v", err)
+	}
+	if len(d.deleted) != 0 {
+		t.Errorf("Close must not delete commands: %v", d.deleted)
+	}
+}