docs: refresh engineering runbooks

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Summary

• Updated engineering, architecture, setup, configuration, command, feature, and contributor docs for current source behavior.
• Documented event-loader registration, dual interaction routing/cooldown constraints, release tag gating, Prisma generation setup, rank-card presence handling, and current regression-test coverage.
• Added docs-index links for release/CI and the full docs index from the root README.

Codepaths Covered

• src/handlers/events.js
• src/events/Guild/interactionCreate.js
• src/events/validations/chatInputCommandValidator.js
• src/events/ready/registerCommands.js
• src/events/ready/registerContextMenus.js
• src/example.config.js
• .github/workflows/release.yml
• src/commands/slash/General/rank.js
• src/utils/rankCardPresenceStatus.js
• tests/*.test.js

Key Knowledge Gaps Addressed

• Removed stale event-loader caveat that contradicted { event, run } registration.
• Corrected release workflow docs from version-diff gating to tag-existence gating.
• Clarified DEV_GUILD_ID command registration, PRODUCTION=false MongoDB URI truthiness, and no Prisma postinstall hook.
• Synced rank command docs and contributor test inventory with source/tests.

Verification

• git diff --check HEAD~1..HEAD
• npm test