Add shell completion generation command

[pollychen-lab]

Summary

• add a generate-shell-completion <SHELL> subcommand that writes completions to stdout
• reuse the existing Clap command definition and clap_complete shell support used by the build script
• add integration coverage for completion generation and invalid shell handling

Fixes #881.

Validation

• cargo fmt --check
• cargo test --test integration_tests shell_completion
• cargo test
• cargo run --quiet -- generate-shell-completion fish > /tmp/... and verified the output contains Fish completion entries for hyperfine
• git diff --check

[chatgpt-codex-connector]

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