task-management skill: 'remind me to' creates a static task, not a time-based reminder

## Problem

The `task-management` skill in the productivity plugin handles "remind me to..." by adding a line to `TASKS.md`. This creates a static task — there's no time-based delivery, no notification, no recurrence. The user has to manually check the file to see their reminders.

In production, this is the #1 gap we hit. Sales reps say "remind me to call Acme Friday at 8am" and expect a notification at that time. Instead they get a bullet point in a markdown file that nobody checks.

## What's needed

Time-based reminders that **deliver themselves** — via Slack DM, email, or calendar notification — when they come due. Key requirements from our production usage:

- Natural language time parsing ("next Tuesday at 2pm", "every weekday at 9am")
- Delivery via ~~chat at the scheduled time
- Recurrence (daily, weekly, weekdays, monthly)
- Cross-team: "remind Sarah to send the proposal" should notify Sarah, not just the sender

## Current workaround

We built a full reminder system outside the plugin framework: database storage, 15-minute cron checker, Slack DM delivery, recurrence handling, and server-side date correction (Claude sometimes generates timestamps with the wrong year — see below).

## PR submitted

PR #140 adds a `proactive-reminders` skill that documents this pattern, including the delivery mechanism, storage schema, date validation, and cross-team reminders. It complements `task-management` — tasks track what needs doing, reminders ensure it gets done on time.

## Production lesson: date validation is critical

Claude's knowledge cutoff means it sometimes generates timestamps with the wrong year (e.g., `2025-03-25` instead of `2026-03-25`). Reminders with past dates fire immediately. We had to add server-side auto-correction:

```
IF remind_at is in the past:
TRY replacing year with current year
TRY replacing year with next year (Dec→Jan edge case)
ELSE reject
```

Any reminder implementation needs this validation layer.