docs(commands): surface existing !remove <position> in help and README

Closes #59. The command was already implemented but not advertised in
!help output or the README command table, so users assumed it was missing.

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

src/bot/instance.ts

@@ -778,6 +778,7 @@ export class BotInstance extends EventEmitter {
       `${p}stop         — Stop and clear queue`,
       `${p}vol <0-100>  — Set volume`,
       `${p}queue        — Show queue`,
+      `${p}remove <pos> — Remove song at position (see ${p}queue)`,
       `${p}mode <seq|loop|random|rloop> — Play mode`,
       `${p}playlist <name or id> — Load playlist by name or ID`,
       `${p}playlist -q <name or id> — Load playlist from QQ Music`,