Document prefix command setup prerequisites

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Summary

• Update prefix command documentation in docs/commands.md, docs/configuration.md, and docs/getting-started.md.
• Clarify that prefix command execution requires both handler.commands.prefix: true and Discord Message Content Intent.
• Document fallback/default prefix behavior and the MongoDB dependency for per-guild custom prefixes.

Codepaths covered

• src/handlers/commands.js loading prefix modules into client.collection.prefixcommands
• src/events/Guild/messageCreate.js gating execution on handler.commands.prefix, reading message.content, and resolving per-guild prefixes
• src/class/ExtendedClient.js requesting GatewayIntentBits.MessageContent
• src/schemas/GuildSchema/guildschemas storage for custom prefixes

Key knowledge gaps addressed

• Prefix modules can load while execution remains disabled by configuration.
• Discord's Message Content Intent is required outside of repo config.
• Custom prefixes depend on MongoDB; installs with database access disabled fall back to handler.prefix.

Verification

• git diff --check
• npm test (48 passing tests)

Prefix command docs verification summary