README presents the unpublished v2.0.0 release as stable

[codeforester]

Problem

README.md currently presents v2.0.0 as an existing release and links directly to the unpublished v2.0.0 GitHub Release. The repository still has no v2.0.0 tag or Release; v1.4.0 remains the last stable release until the guarded GA cutover.

Acceptance criteria

• Keep the top-strip version synchronized with VERSION=2.0.0 while explicitly identifying v2.0.0 as planned/pre-release until GA.
• Replace the unresolved release link with a truthful release-preparation or roadmap link.
• Preserve the v2 release row and Homebrew instructions without implying that GA assets already exist.
• Add validation that prevents an unpublished stable-release URL from being presented as current.
• Link the implementation PR and verify the rendered README after merge.

Parent roadmap: #214
Release gate: #215

This is a documentation truthfulness fix; it does not publish or unlock v2.0.0.