Add /discord link and /discord unlink commands

[liwd190019]

Summary

Implements the first slice of account-linking support from Issue #17 by adding a new /discord command with link and unlink subcommands.

Changes

• Adds /discord link:
  • Requires in-game player sender
  • Requires DB pool configured/available
  • Rejects if player already linked
  • Generates a short link code via existing JDBC helpers and instructs the player to DM it to the bot
• Adds /discord unlink:
  • Requires DB pool configured/available
  • Rejects if player not linked
  • Unlinks via existing JDBC helpers
• Registers the new command in the plugin entrypoint

Files

• src/net/peacefulcraft/cowbot/commands/DiscordCommand.java
• src/net/peacefulcraft/cowbot/CowBot.java

Notes / Follow-ups

• This PR does not yet implement the Discord DM flow to consume the code and finalize linking, or the configurable role grant; intended as a small, reviewable first step toward Issue Discord <-> Minecraft account linking #17.