feat: add shell completion support

[zyaiire]

Summary

Fixes #34

Adds shell completion script generation via a new completion subcommand, powered by clap_complete. Supports bash, zsh, fish, elvish, and powershell.

Changes

• Add clap_complete = "4" dependency to Cargo.toml
• Add Completion { shell } subcommand to the Commands enum
• Generate completions to stdout using clap_complete::generate()

Usage

# Generate completion script for your shell
polymarket completion bash > ~/.local/share/bash-completion/completions/polymarket
polymarket completion zsh  > ~/.zfunc/_polymarket
polymarket completion fish > ~/.config/fish/completions/polymarket.fish

Testing

• cargo build — compiles cleanly
• cargo test — all 49 tests pass
• cargo clippy — no warnings
• Manually verified polymarket completion bash outputs a valid completion script

Notes

• Minimal change: only 3 files touched (Cargo.toml, Cargo.lock, src/main.rs)
• No modifications to existing commands
• Uses the same clap_complete approach suggested in the issue

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Note

Low Risk
Low risk: adds a new completion subcommand and dependency without changing existing command execution paths or any network/auth/data-handling logic.

Overview
Adds shell completion generation to the CLI via a new completion subcommand that outputs clap_complete scripts to stdout.

This introduces the clap_complete dependency (and lockfile updates) and wires Commands::Completion { shell } to call clap_complete::generate() using the existing clap command definition.

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