perf(shell): optimize PowerShell detection to prevent false positives on Windows

[SandroHub013]

This PR refines the PowerShell command detection in packages/nikcli/src/shell/shell.ts on Windows.

Problem

Previously, any command referencing standard environment variables using the $ symbol (such as echo $PATH or export FOO=$BAR) was incorrectly flagged as a PowerShell command, forcing nikcli to spawn powershell.exe under the hood.

Because powershell.exe has high startup latency (0.5s to 2.0s due to .NET CLR and profile loading), this caused a severe lag of 1-2 seconds per command. Moreover, Unix-style commands like export FOO=$BAR would fail completely under PowerShell.

Solution

1. Removed the overly broad /\$(?:\w+|{[^}]+})/ regex from POWERSHELL_MARKERS.
2. Replaced it with a targeted regex matching actual PowerShell automatic variables (e.g., $PSVersionTable, $profile, $true, $false, $null, $args, $input, $Matches).
3. Ensures Unix/Bash commands correctly execute in the preferred fallback shell (such as Git Bash or CMD), speeding up execution by up to 50x.

[github-actions]

Hey! Your PR title perf(shell): optimize PowerShell detection to prevent false positives on Windows doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, nikcli).

See CONTRIBUTING.md for details.

[github-actions]

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Please edit this PR description to address the above within 2 hours, or it will be automatically closed.

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[nikomatt69]

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