Predicate for @mentions

[melvincarvalho]

Summary

Define a predicate for @mentions in chat messages, allowing messages to reference/mention specific users.

Proposed Usage

<#msg-123> sioc:content "Hey @alice, check this out!" ;
           :mentions <https://alice.solidweb.org/profile/card#me> .

Predicate Options

1. schema:mentions - Schema.org, widely used
2. sioc:addressed_to - Already in SIOC vocabulary
3. as:tag with as:Mention type - ActivityStreams approach

Scope

This issue is specifically about the predicate choice. Notification mechanisms (LDN, Solid Notifications, etc.) are out of scope.