Update engineering documentation for release and event flows

[cursor]

Summary

• Updated docs/engineering-guide.md to document current event-loader contracts and release tag-existence gating.
• Updated docs/commands.md and docs/features.md to align /rank info docs with its required user option and rank-card presence fallback behavior.
• Updated docs/contributing.md with release workflow guidance and the current regression-test inventory.

Codepaths Covered

• .github/workflows/release.yml
• src/handlers/events.js
• src/events/Guild/interactionCreate.js
• src/events/Guild/messageCreate.js
• src/commands/slash/General/rank.js
• src/utils/rankCardPresenceStatus.js
• tests/events-handler-shape.test.js
• tests/interaction-cooldown.test.js
• tests/rank-card-presence-status.test.js

Key Knowledge Gaps Addressed

• Removed stale guidance that Guild event modules were not registered by their declared Discord event.
• Clarified releases are gated by missing v<package.json version> tags, not latest-commit version-line diffs.
• Corrected /rank info from optional self-targeting to a required target member.
• Documented why missing/unsupported Discord presence renders as offline in rank cards.

Verification

• git diff --check HEAD~1..HEAD
• npm test (37 tests passing)