Fix Windows PowerShell stderr redirection for pipx specsmith.exe

[tbitcs]

Problem

Running the pipx-installed Windows entry point with PowerShell stderr redirection, for example specsmith kill-session 2>\$null, can fail before the CLI executes with a launcher error involving StandardErrorEncoding. The documented session-start command uses this pattern.

Scope

Identify whether the failure originates in the pipx launcher, console-script wrapper, Python runtime, or PowerShell native-command handling, then make the global pipx installation work without a development venv or alternate shell wrapper.

Acceptance criteria

• specsmith kill-session 2>\$null runs successfully in supported Windows PowerShell versions and preserves the command's normal exit-status behavior.
• Equivalent redirection for a representative normal command works without a launcher failure.
• Regression coverage exercises the installed Windows entry-point behavior, or a narrowly justified executable-launcher equivalent.
• The documented bootstrap command remains valid on Windows.
• The fix does not alter stdout/stderr behavior on Linux or macOS.