#18434: feat(discord): add broadcast group support for multi-agent fan-out

## Summary Port the existing WhatsApp broadcast pattern to Discord guild channels, enabling multiple agents to independently process the same message in a Discord channel. ## Motivation Broadcast groups are a powerful feature for multi-agent setups where specialized agents (coder, reviewer, researcher, etc.) collaborate in the same channel. This is currently supported for WhatsApp but not Discord, despite Discord being a natural fit for project-based multi-agent workflows. ## Changes ### New: `src/discord/monitor/broadcast.ts` - `maybeBroadcastDiscordMessage()` — checks `config.broadcast[channelId]` and fans out to all listed agents - Only applies to guild/channel messages (never DMs) - Rebuilds route per agent with correct `agentId`, `sessionKey`, and `mainSessionKey` - Supports `parallel` (default) and `sequential` strategies - Per-agent error handling — one agent failure doesn't block others - Snapshots guild history before fan-out, clears once after all agents finish ### Modified: `src/discord/monitor/message-handler.ts` - Imports and calls `maybeBroadcastDiscordMessage()` in both flush paths (single-message and debounced-batch) - If broadcast returns `true`, skips normal single-agent processing ### Modified: `src/discord/monitor/message-handler.process.ts` - Extended `processDiscordMessage()` signature with optional `opts` parameter: - `guildHistory?: HistoryEntry[]` — use snapshotted history instead of live map - `suppressGuildHistoryClear?: boolean` — prevent per-agent history clearing during broadcast - Imported `HistoryEntry` type ## Config example Uses the same `BroadcastConfig` type — no schema changes needed. Discord channel IDs work as peer IDs: ```json5 { broadcast: { strategy: "parallel", "123456789012345678": ["coder", "reviewer", "researcher"], "987654321098765432": ["pm", "coder", "qa"], } } ``` ## Testing - All 96 existing Discord monitor tests pass - Zero TypeScript errors - No changes to WhatsApp broadcast, config types, or zod schema <!-- greptile_comment --> <h3>Greptile Summary</h3> Ports WhatsApp broadcast pattern to Discord, enabling multi-agent fan-out in guild channels. Each broadcast agent receives the same message with independent processing, snapshotted history, and per-agent identity via Discord webhooks. **Key changes:** - New `maybeBroadcastDiscordMessage()` function in `src/discord/monitor/broadcast.ts` checks `config.broadcast[channelId]` and fans out to configured agents - Modified `processDiscordMessage()` to accept optional `guildHistory`, `suppressGuildHistoryClear`, and `agentIdentity` parameters for broadcast support - Webhook delivery (`send.webhook.ts` + `webhook-cache.ts`) enables per-agent identity (username/avatar/emoji) for broadcast agents - Guild history is snapshotted before broadcast and cleared once after all agents complete, preventing race conditions - Supports both `parallel` (default) and `sequential` broadcast strategies with per-agent error isolation **Implementation follows WhatsApp broadcast pattern** (`src/web/auto-reply/monitor/broadcast.ts`) with Discord-specific adaptations for webhook delivery and guild channel handling. <h3>Confidence Score: 5/5</h3> - Safe to merge - clean port of existing WhatsApp pattern with comprehensive test coverage and no breaking changes - Implementation faithfully follows the proven WhatsApp broadcast pattern with proper error isolation, history snapshotting, and TypeScript type safety. The webhook delivery feature is well-encapsulated with fallback to standard delivery. All 96 existing Discord tests pass, and the PR includes dedicated webhook delivery tests. No schema changes required since `BroadcastConfig` already supports the pattern. - No files require special attention - implementation is clean and follows established patterns _{Last reviewed commit: 8741759} <!-- greptile_other_comments_section --> _{(4/5) You can add custom instructions or style guidelines for the agent [here](https://app.greptile.com/review/github)!} <!-- /greptile_comment -->