feat(platform): add native runtime support gates

[decode2]

Pre-flight Checks

• I have searched existing issues and this is not a duplicate
• I understand this issue needs status:approved before a PR can be opened

Problem Description

Native Windows ConPTY and supported Ubuntu PTY behavior currently lack explicit, authoritative runtime support gates and deterministic CI receipt coverage.

Proposed Solution

Add a named Windows ConPTY gate, a pinned Ubuntu 24.04 native runtime lane, and deterministic runtime/CI-contract tests with explicit unsupported-host skips. This issue does not add or claim WSL support.

Affected Area

CI/Workflows

Alternatives Considered

Relying only on the existing workspace test jobs does not make platform runtime authority or support boundaries explicit.