Add file sending support for Discord agents (agent -> Discord)

[edspencer]

Summary

Slack-connected agents can generate files and send them to the chat channel via the herdctl_send_file MCP tool (added in #47). Discord-connected agents cannot — they lack the equivalent wiring. The infrastructure is already platform-agnostic; Discord just needs to plug into it.

Current State

• Slack: slack-manager.ts creates a FileSenderContext, wraps it via createFileSenderDef(), and passes it as injectedMcpServers to fleetManager.trigger(). The Slack connector has an uploadFile() method that calls files.uploadV2().
• Discord: discord-manager.ts does not pass injectedMcpServers to fleetManager.trigger(). The Discord connector has no uploadFile() method.

What Needs to Change

1. Add uploadFile() to the Discord connector

Discord.js supports file uploads via channel.send():

await channel.send({
  content: message,
  files: [{ attachment: fileBuffer, name: filename }]
});

Add an uploadFile(params) method to the Discord connector matching the same shape used by the file sender context.

2. Wire up FileSenderContext in discord-manager.ts

Follow the same pattern as slack-manager.ts:

const fileSenderContext: FileSenderContext = {
  workingDirectory: workingDir,
  uploadFile: async (params) => {
    return connector.uploadFile({
      channelId: event.metadata.channelId,
      fileBuffer: params.fileBuffer,
      filename: params.filename,
      message: params.message,
    });
  },
};
const fileSenderDef = createFileSenderDef(fileSenderContext);
injectedMcpServers = { [fileSenderDef.name]: fileSenderDef };

Then pass injectedMcpServers to fleetManager.trigger().

3. No changes to file-sender-mcp.ts

The file sender MCP tool definition is already platform-agnostic. It accepts a FileSenderContext with a generic uploadFile() callback. No changes needed.

Scope

This is a small change — the architecture and tool definition already exist. It's wiring Discord into the same pattern that Slack uses.