vix pack: add --sign=auto|never|required (non-blocking default)

[GaspardKirira]

vix pack: add --sign=auto|never|required (non-blocking default)

Problem

Current signing behavior can be unclear and may block in some environments (CI, non-interactive shells) when minisign prompts for a password.

Goal

Introduce a predictable signing policy similar to npm:

• --sign=auto (default): sign only if minisign + key are available, never fail.
• --sign=never: never sign.
• --sign=required: must sign, fail-fast with clear error messages.

Acceptance criteria

• vix pack defaults to --sign=auto.
• --sign remains as alias of --sign=required.
• --sign=auto never blocks and never fails the pack if signing cannot be performed.
• --sign=required exits with code 1 when minisign/key missing or signing fails.
• Help output documents the modes + env var VIX_MINISIGN_SECKEY.

Notes

Signing output should be explicit in verbose mode:

• show key path used (or "not found")
• show whether minisign was detected