# Account Merge

## Objective

Collapse two duplicate accounts into one without losing work.

A controlled flow that merges a source account into a target account, resolving conflicts across ownership, billing, and content.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Require proof of control over both accounts in the same flow — a verified challenge to each, not just being signed into one.
2. Show the user a merge plan before anything moves: what will be kept, what will be reassigned, what will be discarded, and what cannot be undone.
3. Resolve each conflict class explicitly — workspace memberships, subscriptions, usernames, handles, preferences, API keys. Pick a rule per class and state it in the UI rather than guessing.
4. Reassign owned records to the target account rather than copying them, so history, timestamps, and references stay intact.
5. Run the merge as a resumable job with a recorded plan, so a partial failure can be retried or reversed instead of leaving two half-accounts.
6. Do NOT delete the source account until the merge has been verified. Deactivate it, keep the mapping, and reap it later.
7. Adding an extra sign-in method to one account is owned by Account Linking; send users there when they do not actually have two accounts.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- Both accounts holding paid subscriptions must be handled deliberately — cancel one with credit, or refuse and require billing to be sorted first. Never leave the user paying twice.
- Both accounts being members of the same workspace must collapse to a single membership at the higher role, not two rows.
- Where both accounts own a unique value such as a username, decide which survives and free the other rather than erroring mid-merge.
- Third-party integrations authorised by the source account must be re-authorised or explicitly migrated; tokens scoped to a dead account fail silently later.
- Anything that references the source account by ID — links, mentions, webhooks, exported data — needs a redirect from old ID to new for a defined period.
- A merge must be blocked when either account is suspended or mid-deletion, and it must be single-flight so two merges cannot run against the same pair.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] Control of both accounts is proven before the merge begins.
- [ ] A plan showing kept, moved, and discarded data is confirmed by the user first.
- [ ] Owned records are reassigned, not duplicated, and their history survives.
- [ ] Duplicate memberships and unique-value conflicts resolve to a single defined outcome.
- [ ] A partial failure can be retried or rolled back from the recorded plan.
- [ ] The source account resolves to the target for a defined period after the merge.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.