Release: finalize the migration guide, test the AI prompt, write the 1.0.0 changelog

Part of #120 (Phase 5 — Release).

Scope

Finalize the consumer-facing release documentation, once the full break surface is known and soaking.

• docs/migration-v0.8-to-v1.0.md — review end to end. Per-PR entries have been accumulating throughout the cycle by design, so this is a coherence and completeness pass, not a writeup from scratch. Confirm the scope decision holds: breaking changes plus behavioral changes that can require action on upgrade.
• Test the AI migration prompt. The guide's top-of-file prompt is a v1.0.0 deliverable and has never been exercised. Run it against a real sample consumer build.ps1 with an agent and fix what it gets wrong. An untested prompt that produces a broken build.ps1 is worse than no prompt.
• CHANGELOG.md — write the 1.0.0 entry, summarizing the breaks and linking to the migration guide.

Gate

Blocked on every other v1.0.0 pull request being merged — the break surface must be complete before the guide can be called finished. Re-gated 2026-09-04: the preview and rc were dropped from the plan, so the gate is now the merge of #224, #225, #226, #227 and #228. This issue must close before 1.0.0 is tagged.

Done when

The migration guide is complete, the AI prompt has been verified against a sample consumer build, and the changelog entry is written.

Xem lại docs/migration-v0.8-to-v1.0.md sau khi các gated pull requests được merge, sau đó chạy AI prompt ở đầu tệp đó trên một build.ps1 thực của consumer. Xác nhận rằng hướng dẫn bao quát các thay đổi hành vi gây breaking và liên quan đến việc nâng cấp, kiểm tra để bảo đảm prompt không tạo ra một build bị hỏng, và viết mục 1.0.0 trong CHANGELOG.md có liên kết đến hướng dẫn.