feat: refine shell completion UX

[francoischalifour]

The CLI already had a strong altertable completion install flow that writes shell completion files and updates shell startup files where needed. The weaker part was the default altertable completion behavior: it generated raw completion script output, which is useful for manual installs but surprising for users who run the command looking for setup guidance.

This branch makes completion setup more task-oriented:

• altertable completion install remains the primary path.
• altertable completion generate <shell> becomes the explicit manual/raw-script path.
• altertable completion <shell> remains supported as a compatibility alias.
• Bare altertable completion becomes user-facing guidance instead of dumping a script.

Preview

Proposed Solution

This change splits the completion command into clearer entry points:

altertable completion
├── install [bash|fish|zsh]
├── generate bash|fish|zsh
└── bash|fish|zsh  # compatibility raw-script aliases

The root completion command now behaves according to context:

• In an interactive terminal, it opens a small install-focused prompt using the existing @clack/prompts abstraction.
• In non-interactive mode, it prints concise setup guidance.
• In JSON/agent mode, it emits the same guidance as structured data.
• It no longer prints raw shell scripts unless the user explicitly invokes generate <shell> or a compatibility shell alias.

The implementation keeps raw script generation on leaf commands only. The generate node is a plain group command, while generate bash, generate zsh, and generate fish own the raw stdout behavior. This keeps operation metadata accurate and avoids unnecessary usage-renderer or command-context changes.