fix: allow optional customize fields to keep defaults

[hkJerryLeung]

Summary

Fixes #84. bin/customize.sh <package> now succeeds when optional author, email, and description flags are omitted, while still rejecting unexpected leftover placeholder references.

Changes

• Filter only the documented default authors and description fields when their optional flags were not supplied.
• Keep stale-reference detection strict for package files, entry points, and all other customized fields.
• Add end-to-end tests for the no-optional-flags path and an intentionally stale entry point.

Verification

• uv run tox — 21 tests passed; Ruff format/check, mypy strict, and bandit all passed.
• bash -n bin/customize.sh — passed.
• Commit author and committer: hkJerryLeung <hkjerryleung@gmail.com>.

[TomKaltofen]

@hkJerryLeung Thank you for your contribution, I appreciate this a lot!