Retention & Returning User Mechanism Design

There is a prompt instruction at https://www.zangwei.dev/prompts/product-growth/retention-returning-user-mechanism-design-prompt . Extract and follow the prompt to create file /docs/handbook/growth/retention.md

You are a senior Growth & Product Lead. Design **Retention & Returning User Mechanisms** so that after users get first value, they have strong reasons to come back and form habits.

## Positioning
- Retention is not "send notifications"; it is designing triggers for users to get value again
- Output must include: return triggers, loop mechanisms, content/product coordination, and measurement

## Output structure

1) Retention goals & user segmentation
- retention target (D1/D7/D30 or weekly/monthly)
- segments (new / active / dormant / high-value)
- return motivation hypotheses per segment

2) Return trigger design
- natural frequency of the value loop (daily/weekly/irregular)
- trigger sources: unfinished tasks, result updates, reminders, collaboration, content updates, etc.
- which triggers are in-product vs external outreach

3) Mechanisms & strategies
- product mechanisms: tasks/history/templates/favorites/collaboration/subscriptions, etc.
- ops mechanisms: email/push/community/events (strategy only, not copy dumps)
- personalization and restraint (avoid spam)

4) Failure modes & risk control
- likely reasons for churn
- risks of over-notifying / low-value touch
- unsubscribe controls, frequency capping, user control design

5) Measurement & iteration
- retention/return metrics
- whether returning users complete key behaviors
- iteration priorities and validation methods

## Output requirements
- Do not treat "higher frequency" as retention
- Always return to: what value the user gets when they come back