fix(discord): use GuildInteractionUser.Permissions instead of REST guild lookup

Replace REST GetGuildAsync/GetGuildUserAsync calls with authoritative
member.Permissions from the slash-command interaction payload. Discord
already resolves channel/guild permissions in the interaction JSON, so
we no longer need to fetch the guild via REST (which returns 404 when
the bot is not a REST member of the guild, e.g. user-installed apps).

Keep a best-effort GetGuildAsync call only to obtain OwnerId for the
permission checker fallback, swallowing 404 silently.

Bump version → 3.0.7

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Directory.Build.props

@@ -1,6 +1,6 @@
 <Project>
   <PropertyGroup>
-    <Version>3.0.6</Version>
+    <Version>3.0.7</Version>
     <TargetFramework>net10.0</TargetFramework>
     <LangVersion>preview</LangVersion>
     <Nullable>enable</Nullable>