fix: accept pwsh for completion shell

[mi6886]

Fixes PlatformNetwork/bounty-challenge#53188.

This updates the completion shell parser so cortex completion pwsh is accepted as an alias for PowerShell, matching the existing shell-detection helpers that already map pwsh to Shell::PowerShell.

Changes:

• Replaces the strict clap_complete::Shell value enum parser for the completion positional argument with a small Cortex-owned parser.
• Preserves the existing shell names: bash, zsh, fish, powershell, and elvish.
• Adds pwsh as a PowerShell alias.
• Adds regression coverage for cortex completion pwsh and unsupported shell rejection.

Verification:

• git diff --check

Note: I could not run the focused Rust tests locally because this environment does not currently have cargo installed in PATH.

[greptile-apps]

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