docs(constructive-features): add EventReferral and multi-level MLM to feature catalog

[pyramation]

Summary

Adds two rows to the Events & Achievements section of the constructive-features skill (the feature catalog/router):

• EventReferral (attribute events to inviters) — gated by EventReferral node + invites_module
• Multi-level referral chains (MLM) — gated by EventReferral node + max_depth (2–10)

Both point to constructive-sdk-events which was updated with full EventReferral docs in PR #133.

Review & Testing Checklist for Human

• Verify the two new rows are correctly placed in the Events & Achievements table

Notes

Docs-only. Follows up on PR #133 which was merged before this commit was pushed.

Link to Devin session: https://app.devin.ai/sessions/2ad5d38ab24e4d43ba283eb9ae8b5c81
Requested by: @pyramation

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