Add CLI shell completion (bash/zsh/fish) and CLI recipes/docs

[MicheleBonus]

Motivation

• Improve discoverability and ergonomics for users migrating from TUI/GUI to CLI by providing shell completion and high-value CLI workflows.
• Provide one-step recipes and documentation so CLI users can bootstrap manifests, validate packages for publication, and export publication-ready artifacts.

Description

• Add a completion subcommand that prints generated shell completion scripts for bash, zsh, and fish via ambermeta completion {bash,zsh,fish} and implement _completion_script and _completion_command in ambermeta/cli.py.
• Wire the completion subparser into build_parser() and dispatch in main(), and include completion-related help in the autogenerated CLI docs flow by updating scripts/export_cli_help.py to include the new block.
• Document how to install/activate completions in README.md and docs/cli.md, add a dedicated Completion Command section and shell installation snippets, and add docs/recipes.md containing three high-value workflows: "from raw directory to manifest", "validate all files before publication", and "export methods summary + stats CSV in one command".
• Add tests/test_cli_completion.py to validate parser wiring and that generated scripts contain expected tokens.

Testing

• Ran pytest tests/test_cli_completion.py tests/test_cli_plan.py tests/test_cli_validate.py, and all tests passed (7 passed).
• Ran the docs generation checker python scripts/export_cli_help.py --check, which reported docs/cli.md is up to date.

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Codex Task