refactor(pack): extract create argument parsing

[cedric013]

Summary

• extract pack create argument parsing into focused helpers
• extract tail-end setup orchestration blocks into named helpers
• preserve existing permissive CLI parsing behavior

Validation

• rustfmt --edition 2024 --check src/setup/with_options.rs src/cli/pack_cmd/package.rs
• git diff --check -- src/setup/with_options.rs src/cli/pack_cmd/package.rs
• cargo check --bin lean-ctx --quiet

Refs #1277