Migrate quiltx bucket add workflow into quilt3

[drernie]

Summary

Move the customer-facing quiltx bucket add workflow into quilt3, while retaining quiltx bucket add as a thin compatibility shim.

quilt3.admin.buckets.add() already provides registry CRUD. The migration should cover the surrounding AWS workflow: safe S3 bucket-policy merging, SNS policy/topic setup, notification conflict and drift checks, --no-preflight registration, and post-add verification.

Authoritative implementation and tests:

• Core bucket workflow
• CLI orchestration
• Tests

Acceptance criteria

• Equivalent bucket-add API/CLI behavior is available from quilt3.
• The quiltx command delegates to quilt3 without breaking existing automation.
• Existing policy, notification-conflict, drift, no-preflight, and verification tests migrate.
• Add moto coverage for bucket-policy merging and notification-conflict behavior.