feat: command plugin/extension system (registerCommand API)

[cpfiffer]

Summary

Add a registerCommand(name, handler) API that lets local deployments add custom commands without modifying core files.

Motivation

Community user (bibbs) added a runtime /config command to adjust jitter, debounce, and heartbeat settings without restarting their 3 Raspberry Pi instances. This requires patching bot.ts and commands.ts on every upstream rebase.

More broadly, any deployment-specific command (debug tools, status checks, integration triggers) currently requires forking core files. A plugin API would eliminate this friction for all downstream users.

Current Workaround

• Add 'config' to the COMMANDS array in commands.ts
• Add case 'config' in handleCommand in bot.ts
• Parse subcommands (jitter, debounce, heartbeat) and modify in-memory settings
• Also added reschedule(intervalMinutes) and getIntervalMinutes() to HeartbeatService for runtime heartbeat adjustment

Proposed API

// In a plugin file or config-referenced module
export default function register(bot: LettaBot) {
  bot.registerCommand('config', async (args, context) => {
    // handle subcommands
  });
}

Or config-driven:

features:
  commands:
    - name: config
      module: ./plugins/config-command.ts

Implementation Considerations

• Commands currently live in a static COMMANDS array and a switch statement in handleCommand
• Plugin commands would need access to bot internals (config values, services like HeartbeatService)
• Should validate no collisions with built-in commands
• The heartbeat reschedule() method is independently useful and could ship as a built-in /heartbeat interval <N> subcommand regardless of the plugin system

Context

This is one of 5 issues filed from a community user's multi-bot deployment feedback.