docs(windows): establish branch-specific top-level Windows guidance #4755

User Story

As the maintainer preparing OpenShell's native Windows release, I need the top-level documentation to guide readers to the Windows implementation and keep the windows branch as the Windows release source when shared docs also appear on main.

Problem Statement

The top-level README routes Windows readers only to WSL, and build, test, CI, and agent guidance do not consistently identify windows as the implementation and release branch.

Impact / Why This Matters

Readers can choose the wrong checkout, installer, or validation lane. Looking through source code and branch history is the current workaround, which is insufficient for a release entry point.

Description

Add a top-level Windows guide and reconcile the platform-relevant root documents, published entry points, and contributor skills on the windows branch. Explain native MXC architecture, the four executables and Z3 runtime, x64/ARM64 validation, and existing qualification limits. Pin Windows implementation links to windows.

Context

This task covers branch placement and top-level navigation. Related implementation-detail corrections are already tracked by #4645 and PR #4646; this task does not replace or close that work. A separate documentation-only PR will carry main-appropriate references to main without moving Windows code.

Acceptance Criteria

• Root technical docs describe native Windows and identify windows as its implementation and release branch.
• Windows setup, source, and task links remain branch-specific when docs appear on main.
• Native MXC is distinguished from WSL and from the main release installer.
• Existing runtime limitations and validation boundaries remain explicit.
• Changes are documentation-only and pass applicable documentation checks.