feat: add shell completion support

[mvanhorn]

Summary

Adds a polymarket completions <shell> subcommand that generates shell completion scripts using clap_complete.

Fixes #34

Usage

# Bash
polymarket completions bash >> ~/.bashrc

# Zsh
polymarket completions zsh >> ~/.zshrc

# Fish
polymarket completions fish > ~/.config/fish/completions/polymarket.fish

Supported shells: bash, zsh, fish, elvish, powershell.

Changes

File	Change
Cargo.toml	Add clap_complete = "4" dependency
src/main.rs	Add Completions subcommand with Shell argument and clap_complete::generate handler

Test plan

• cargo fmt --check passes
• cargo clippy -- -D warnings passes
• cargo test — all 131 tests pass (82 unit + 49 integration)
• polymarket completions bash outputs valid completion script
• polymarket completions zsh outputs valid completion script

This contribution was developed with AI assistance (Claude Code).

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Note

Low Risk
Adds an optional CLI subcommand and a dependency; no changes to trading/auth flows or data handling.

Overview
Adds a new polymarket completions <shell> subcommand that prints shell completion scripts to stdout via clap_complete::generate.

Wires in the new clap_complete dependency (and lockfile updates) and updates CLI parsing to expose Shell as an argument for completion generation.

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