Feature/shell completion

[404Simon]

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Description

Adds a completion command that generates shell completion scripts for bash, zsh, and fish, enabling tab-completion for all pangolin CLI commands and flags.

How to test?

# Test bash completion generation
pangolin completion bash
# Test zsh completion generation  
pangolin completion zsh
# Test fish completion generation
pangolin completion fish
# Verify completions work (bash example)
source <(pangolin completion bash)
pangolin <TAB><TAB>

[Copilot]

Pull request overview

This PR adds shell completion support for the Pangolin CLI, enabling tab-completion for bash, zsh, and fish shells. The implementation provides a custom completion command that generates appropriate completion scripts for each supported shell.

Key Changes

• Added custom completion command with subcommands for bash, zsh, and fish
• Updated github.com/fosrl/newt dependency from v0.0.0 to v1.8.0
• Added comprehensive documentation for shell completion setup

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
cmd/completion/completion.go	New completion command implementation that generates shell-specific completion scripts
cmd/root.go	Integrates completion command into root CLI and removes default hidden completion options
go.mod	Updates newt dependency to v1.8.0 and moves spf13/pflag to indirect dependencies
go.sum	Adds dependency checksums for newt v1.8.0 and updates google/go-cmp to v0.7.0
docs/pangolin_completion.md	New documentation with installation instructions for all supported shells
docs/pangolin.md	Updates root documentation to reference new completion command

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