feat: in the shell tool, enable fish/nu shells and extend dynamic shell naming to all POSIX shells

[ariane-emory]

What does this PR do?

Resolves anomalyco#4866

Dev recently refactored the Bash tool into a generic Shell tool with shell-aware prompts for PowerShell and cmd. However, two gaps remain:

1. Fish and nu shells are blacklisted — dev still rejects these shells even though they work fine with the existing -c command flag.
2. Non-PowerShell shells still get hardcoded "bash" language — dev dynamically names PowerShell and cmd in the tool description, but falls back to "bash command" / "bash commands" / "git bash commands" for all other shells.

This PR fixes both issues:

• Removes the shell blacklist, allowing fish and nu to be selected and used
• Extends dynamic shell naming (${name}) to the default POSIX shell profile, so fish users see "fish command", zsh users see "zsh command", etc.
• Adds tests verifying shell detection, dynamic description language, and acceptance of fish/nu shells

Benefits

• Fish and nu users can now use their preferred shell without opencode falling back to bash
• The model is told the actual shell name (e.g., "fish", "zsh", "nu") instead of always seeing "bash", reducing the "shell rediscovery loop" where the model tries bash syntax on a non-bash shell
• Fewer failed shell tool calls and wasted context on error messages