fix: improve gateway intents and auth error messages

[masami-agent]

Summary

Closes #116

Catch DisallowedGatewayIntents and InvalidAuthentication errors from the Discord gateway with actionable error messages instead of cryptic serenity errors that cause CrashLoopBackOff with no guidance.

Changes

File	Change
src/main.rs	Replace bare client.start().await? with match block catching DisallowedGatewayIntents and InvalidAuthentication
README.md	Add prerequisites section documenting required Discord Developer Portal settings

Error messages

DisallowedGatewayIntents:

Discord rejected privileged intents. Enable MESSAGE CONTENT INTENT at: https://discord.com/developers/applications → Bot → Privileged Gateway Intents

InvalidAuthentication:

Discord rejected bot token. Verify your bot_token in config.toml is correct and has not been reset.

What is NOT changed

Per discussion in #116:

• Intents are not made configurable in config.toml — openab requires MESSAGE_CONTENT to function
• Only the two most common DX footguns are caught — not every possible serenity error

Testing

• Verified serenity::Error::Gateway(GatewayError::DisallowedGatewayIntents) and GatewayError::InvalidAuthentication exist in serenity 0.12.5
• Issue confirmed reproducible on 0.7.2-beta.3 (see Hardcoded privileged gateway intents cause CrashLoopBackOff with no actionable error message #116 comment)