feat: rewrite as moderation bot (#11)

## Summary

- Replaces the old AI companion bot with a full Discord moderation system
- Adds 8 slash commands: `warn`, `mute`, `unmute`, `kick`, `softban`, `ban`, `unban`, `prune`
- Adds logging for member join/leave, activity (messages, threads, voice), and moderation actions
- Audit log integration captures manual bans, kicks, timeouts, and unbans
- All applicable actions post sanctions to the Hikari sanction API
- All commands are ephemeral, use Components v2, and enforce permission + role hierarchy checks

## Test plan

- [ ] Run `pnpm register` to register all 8 commands to the guild
- [ ] Verify each command appears in Discord and is only visible to members with the appropriate permissions
- [ ] Test each command against a valid target and confirm mod log entry, DM notification, and sanction record
- [ ] Test each command against an invalid target (equal/higher role, self, bot) and confirm correct error response
- [ ] Perform a manual ban, kick, and timeout in the Discord UI and confirm audit log handler picks them up
- [ ] Perform a manual unban and confirm it logs correctly without creating a sanction
- [ ] Verify join/leave messages appear in the welcome log channel
- [ ] Verify message edits, deletes, thread events, and voice state changes appear in the activity log channel

✨ This issue was created with help from Hikari~ 🌸

Reviewed-on: #11
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

src/events/guildMemberRemove.ts

@@ -0,0 +1,55 @@
+/**
+ * @copyright nhcarrigan
+ * @license Naomi's Public License
+ * @author Naomi Carrigan
+ */
+
+import {
+  ChannelType,
+  type GuildMember,
+  type PartialGuildMember,
+} from "discord.js";
+import { channelConfig } from "../config/channels.js";
+import { memberMessage } from "../utils/components.js";
+import { logger } from "../utils/logger.js";
+
+/**
+ * Logs a member leave event to the welcome log channel.
+ * @param member - The guild member who left (may be partial if uncached).
+ * @returns A promise that resolves when the event has been logged.
+ */
+export const onGuildMemberRemove = async(
+  member: GuildMember | PartialGuildMember,
+): Promise<void> => {
+  try {
+    const rawChannel = member.client.channels.cache.get(
+      channelConfig.welcomeLog,
+    );
+
+    if (rawChannel?.type !== ChannelType.GuildText) {
+      return;
+    }
+
+    const joinedAt = member.joinedAt === null
+      ? "Unknown"
+      : member.joinedAt.toLocaleDateString("en-GB", {
+        day:   "2-digit",
+        month: "short",
+        year:  "numeric",
+      });
+
+    const fields = [
+      `**User**: ${member.user.username} (\`${member.id}\`)`,
+      `**Joined**: ${joinedAt}`,
+    ].join("\n");
+
+    await rawChannel.send(memberMessage("leave", fields));
+  } catch (error) {
+    await logger.error(
+      "Failed to log member leave",
+      error instanceof Error
+        ? error
+        : new Error(String(error)),
+    );
+  }
+};