feat(security): commandsSyncKey is a required channel-config field (Guild C-2)

The slash-command sync secret now comes from
channels.fabric.commandsSyncKey (configSchema marks it required) and
is no longer read from FABRIC_COMMANDS_SYNC_KEY env. command-sync
resolves it from config and threads it into client.syncCommands;
when absent, sync is skipped with a clear warning. README updated.

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

dist/fabric/src/accounts.js

@@ -7,6 +7,12 @@ const DEFAULT_CENTER = 'http://localhost:7001/api';
 function section(cfg) {
     return cfg.channels?.fabric ?? {};
 }
+// The commands-sync shared secret (channel-level only). Empty string when
+// unconfigured — callers decide how to handle (slash-command sync is then
+// rejected by the guild).
+export function resolveCommandsSyncKey(cfg) {
+    return (section(cfg).commandsSyncKey ?? '').trim();
+}
 export function listFabricAccountIds(cfg) {
     const accts = section(cfg).accounts ?? {};
     const ids = Object.keys(accts);