[FEATURE] Mentions, inbox, and notification center v1

[aernw]

Objective

Give users a visible collaboration inbox inside the product by greenlight.

Scope

• notification center or inbox surfaces
• mention-aware interaction states in review and comment flows
• clear read/unread and destination behavior for important collaboration events

Out of Scope

• [Features explicitly not included]
• [Future enhancements to defer]

Acceptance Criteria

• users can see collaboration events without relying on external channels
• mentions and notifications feel like a real product capability by greenlight

Dependencies

Blocked by: (none)

Blocks: (none)

Success Metrics

• [How to measure success - optional]

Roadmap Context

Milestone: M5 - Greenlight Build
Parent workstream(s): #161, #210