Use PowerShell by default for legacy Windows recipes

[leynos]

Goal

Make PowerShell the default interpreter for Netsuke legacy command, ordered command-list, and script recipes on Windows. Keep Git Bash as an explicit opt-in compatibility route rather than making it the default Windows runtime.

Context

Issue #599 establishes the v0.1.x Windows legacy-recipe contract and native PowerShell launch smoke coverage. Ninja passes Windows command strings directly to CreateProcess, so Netsuke must explicitly select the recipe interpreter; the shell launching netsuke.exe must not decide it accidentally.

Requirements

• Default legacy recipe execution to PowerShell on Windows.
• Define explicit PowerShell semantics for scalar commands, ordered lists, scripts, quoting, paths, environment variables, and exit-code propagation.
• Retain a documented, explicit Git Bash compatibility option for POSIX legacy manifests.
• Prove both the selected interpreter and a PowerShell-launched end-to-end path.
• Keep the v0.2.0 structured-command design in RFC RFC: structured command blocks and argv templates #573 as the longer-term route away from shell-string ambiguity.

Relationship

• Follow-on architecture requirement discovered while implementing Gate v0.1.0 with native Windows recipe execution and a documented shell contract #599.
• The Gate v0.1.0 with native Windows recipe execution and a documented shell contract #599 release canary must follow this default rather than requiring Git Bash.