docs: document event loader contract

[cursor]

Summary

• Updated docs/engineering-guide.md to replace the stale event-loader caveat with the current contract for validations, function exports, and { event, run } modules.
• Updated docs/architecture.md to clarify interaction validation routing, Guild interactionCreate fallback handlers, and double-reply guard expectations.
• Updated docs/contributing.md with event-handler authoring examples and refreshed test coverage notes.

Codepaths Covered

• src/handlers/events.js
• src/events/validations/**
• src/events/Guild/**
• tests/events-handler-shape.test.js
• Related command/interaction regression tests listed in docs/contributing.md

Knowledge Gaps Addressed

• How direct src/events folders are registered.
• When to use function exports versus { event, run } exports.
• Why fallback interactionCreate handlers need interaction.replied || interaction.deferred guards.
• Which tests should be run after changing the event loader or interaction routing.

Validation

• npm test passed: 37 tests, 37 passing.