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.

Review docs/migration-v0.8-to-v1.0.md after the gated pull requests are merged, then run its top-of-file AI prompt against a real sample consumer build.ps1. Confirm the guide covers breaking and upgrade-relevant behavioral changes, verify the prompt does not produce a broken build, and write the 1.0.0 entry in CHANGELOG.md linking to the guide.