feat(guilds): /guild, the module's own chat command

The first command through `api.registerSlashCommands` (MODULE_API 1.6.0,
TEAMS.md §7.1). The definition and the handler both live here; the bot pulls the
definition and runs no line of this module.

`/guild` and not `/team`, deliberately. Core does not own the word for a Team —
that is what deleted its Team pages in phase 3 — so it does not publish the noun
in a channel either. Core ships the dispatcher and zero commands.

The audience rungs are re-resolved in the handler rather than assumed: a shard
that gates guilds to staff does not become public because the question arrived
over Discord. The provider's own staleness guard is honoured too, so a stale
board answers "not connected" instead of reporting what it still holds, and
`resolveUserId` is exported rather than copied so "linked" means here what it
means on the roster.

Co-Authored-By: Claude <noreply@anthropic.com>

server/test/_fakes.js

@@ -96,6 +96,7 @@ function fakeApi() {
     streams: null,
     legs: [],
     teamProvider: null,
+    slashCommands: [],
     hooks: {},
   }
   const called = new Set()
@@ -112,6 +113,10 @@ function fakeApi() {
     // deployment — a second registration is a collision there, so it has to be
     // one here too, or this suite would pass a shape core rejects at load.
     registerTeamProvider(provider) { once('registerTeamProvider'); record.teamProvider = provider },
+    // MODULE_API 1.6.0, live since phase 7. `once` for the same reason core
+    // takes it: a second call is a module changing its mind halfway through
+    // register(), which core rejects.
+    registerSlashCommands(commands) { once('registerSlashCommands'); record.slashCommands = commands },
     onBoot(fn) { once('onBoot'); record.hooks.onBoot = fn },
     onShutdown(fn) { once('onShutdown'); record.hooks.onShutdown = fn },
   }