Finish the GUILD_ID -> DISCORD_GUILD_* migration: .env.example crashes the discord-bot at boot #155

.env.example and the discord-bot disagree about a required variable, so provisioning a service from the example file fails at startup.

• .env.example:54 — # Guild (server) IDs per community — replaces the old single GUILD_ID, then defines DISCORD_GUILD_COPILOTKIT and DISCORD_GUILD_AGUI. No GUILD_ID line is provided.
• apps/discord-bot/src/config.ts:7 — guildId: requireEnv('GUILD_ID').

requireEnv throws on a missing value, so a bot configured from .env.example exits with Missing required environment variable: GUILD_ID before it connects. docs/deployment.md:79 also still lists GUILD_ID in the Discord env group, so the docs and the example contradict each other as well.

Either the migration is unfinished (the bot should read the per-community vars) or the comment is wrong (GUILD_ID is still required and belongs in the example). Both readings are defensible from the code, which is the problem — pick one and make the three places agree:

1. apps/discord-bot/src/config.ts
2. .env.example (lines 53-57, and 79's counterpart in the docs)
3. docs/deployment.md:79

Verified during CR of #150; unrelated to that PR's subject, hence a separate issue. Directly relevant to the Slack/Discord deploy in #62/#63.