feat: add bash and fish shell completions (currently only zsh and pwsh)

[Neftedollar]

Problem

multiagent-setup completions currently supports only zsh and pwsh. Users on bash or fish get:

Unsupported shell 'bash'. Supported: zsh, pwsh

Expected

Support for the two remaining popular shells:

• bash — eval "$(multiagent-setup completions bash)" or append to ~/.bashrc
• fish — multiagent-setup completions fish | source or save to ~/.config/fish/completions/multiagent-setup.fish

Implementation notes

• Add Templates/tools/completions.bash and Templates/tools/completions.fish
• Register both as embedded resources in MultiagentSetup.csproj
• Add ["bash"] = "tools/completions.bash" and ["fish"] = "tools/completions.fish" to CompletionsCommand.ShellToResource
• Update --help text and PrintUsage() in Program.cs
• Update both zsh and pwsh completion scripts to list bash and fish as valid shells for completions subcommand
• Add tests: CompletionsCommand_Bash_ReturnsScript, CompletionsCommand_Fish_ReturnsScript