docs(extension): record setup wizard design

[xiaoland]

What changed

Add the complete extension-setup-wizard task packet covering:

• working protocol and current-system evidence
• accepted product boundaries and Twitter vertical-slice behavior
• Extension config/state/Source authority decisions
• Core callback, state, Twitter protocol, Web setup, and delivery HLD
• reviewed implementation plan, readiness review, executed result, verification, cleanup, and handoff status

Why

The feature crosses Registry, Core, Client, Python wheel, and Module Federation boundaries. The packet preserves the reasoning and review evidence without moving deployment installation state or Peer runtime authority into the Registry.

Source implementation is tracked separately in:

• feat(extension): add Twitter setup wizard backend core-py#52
• feat(extension): add Twitter setup wizard UI client-web#68 (depends on Core #52)

Impact

This PR changes task documentation only. It does not change the Registry service, publish a Release, deploy resources, or run the deferred real-X black-box acceptance.

Verification

• packet result records Core's 241 passing tests
• packet result records Client's 21 Vitest files / 88 passing tests and all builds
• packet result records exact Core database contract v3 verification
• task-owned runtime/container/volume/network and local credential artifacts were removed
• git diff --check