Add shell completion (bash, zsh, fish)

[ajianaz]

Description

Add shell completion generation for cora-cli to improve developer experience.

Acceptance Criteria

• cora completion bash generates bash completions
• cora completion zsh generates zsh completions
• cora completion fish generates fish completions
• Completions cover all commands, subcommands, and flags
• Document completion setup in README

Implementation Notes

Use clap_complete crate which integrates seamlessly with clap.