Add change spec: email-based conversation matching

Replace thread-ID-based conversation matching with email-address-based
matching for more reliable conversation continuity. Key changes:

- One conversation per email address (simpler model)
- No data expiration (conversations persist indefinitely)
- Post-completion support (questions and registration updates)
- Versioned storage for registration updates (audit trail)
- Admin notifications for registration changes

This addresses the gap where parents sending new emails (instead of
replying) would lose their registration progress.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

openspec/changes/email-based-conversation-matching/specs/registration-notifications/spec.md

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+## ADDED Requirements
+
+### Requirement: Notify admin on registration updates
+The system SHALL send an email notification to the admin when an existing registration is updated, including details of what changed.
+
+#### Scenario: Update notification sent
+- **WHEN** a parent updates their completed registration
+- **THEN** the admin SHALL receive an email notification
+
+#### Scenario: Update notification content
+- **WHEN** sending an update notification
+- **THEN** the notification SHALL include:
+  - Child name and registration ID
+  - What changed (field name, old value → new value)
+  - When the change was made
+  - Version number (e.g., "Version 2 of 2")
+
+#### Scenario: Update notification routing
+- **WHEN** sending an update notification
+- **THEN** the notification SHALL be routed to the same recipients as the original registration (based on playgroup type)
+
+### Requirement: Distinguish new vs update notifications
+The system SHALL clearly distinguish between new registration notifications and update notifications in the email subject and content.
+
+#### Scenario: New registration subject
+- **WHEN** sending a notification for a new registration
+- **THEN** the subject SHALL be "New Registration: [Child Name] for [Playgroup Type]"
+
+#### Scenario: Update notification subject
+- **WHEN** sending a notification for a registration update
+- **THEN** the subject SHALL be "Registration Updated: [Child Name]"
+
+## MODIFIED Requirements
+
+### Requirement: Email reminders for incomplete registrations
+The system SHALL send reminder emails for incomplete registrations, but SHALL NOT threaten data deletion since data no longer expires.
+
+#### Scenario: Reminder content without expiration warning
+- **WHEN** sending a reminder for an incomplete registration
+- **THEN** the reminder SHALL encourage completion but SHALL NOT mention data expiration or deletion
+
+#### Scenario: Reminder schedule unchanged
+- **WHEN** an incomplete registration exists
+- **THEN** reminders SHALL be sent at Day 3, Day 10, and Day 25 after last activity
+
+#### Scenario: Reminders stop after completion
+- **WHEN** a registration is completed
+- **THEN** no further reminders SHALL be sent for that conversation