feat(thread-discovery): implement separate endpoint for fetching threads and update channel loading logic

docs/superpowers/plans/2026-05-13-separate-thread-discovery.md

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+# Separate Thread Discovery Implementation Plan
+
+> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.
+
+**Goal:** Keep channel listing fast and move expensive active/archived thread discovery to a separate endpoint loaded asynchronously by the homepage.
+
+**Architecture:** `VoiceController` exposes cache-only channels and network-backed threads separately. `webserver.ts` adds `/api/guilds/:guildId/threads`. `public/index.html` loads channels first, then appends thread options after thread endpoint returns.
+
+**Tech Stack:** TypeScript, discord.js-selfbot-v13, Express, vanilla JS.
+
+---
+
+### Task 1: Add thread discovery method
+
+**Files:**
+- Modify: `src/voiceController.ts`
+
+- [ ] Add `listThreads(guildId)` that fetches active and archived threads per parent text channel.
+- [ ] Keep `listWatchableChannels` cache-only.
+- [ ] Verify typecheck.
+
+### Task 2: Add thread API endpoint
+
+**Files:**
+- Modify: `src/webserver.ts`
+
+- [ ] Add `GET /api/guilds/:guildId/threads`.
+- [ ] Return thread summaries.
+- [ ] Verify typecheck.
+
+### Task 3: Update homepage dropdown loading
+
+**Files:**
+- Modify: `public/index.html`
+
+- [ ] `loadChannels` fetches `/channels` first and renders immediately.
+- [ ] Then fetches `/threads` async and appends thread options.
+- [ ] Verify typecheck/tests.