docs: update README status badge from pre-release to stable

[millerjp]

Summary

Badge shows status-pre--release-orange but body declares syncmap stable from v1.0.0. The badge is now actively misleading.

Scope

Change README hero badge to status-stable-brightgreen or similar. Bump CHANGELOG entry if needed. Same for any other doc that describes the project as pre-release.

Acceptance criteria

1. Hero badge reads "stable" (or "v1.0.0").
2. No reference to pre-release status remains in README/CHANGELOG/llms.txt.

Source: docs-writer.