Read and set a package's top-level owner

[dduugg]

Fixes #49. Thank you to @oskarpearson for the clear report.

Summary

ParsePackwerk::Package had no way to read or change the top-level owner key, even though write_package_yml! already orders it among the canonical keys. package.owner raised NoMethodError, and package.with(owner: "x") raised ArgumentError, because T::Struct#with only accepts declared props. The only way in was editing package.config['owner'] by hand.

• package.owner returns the top-level owner as a String, or nil.
• package.with(owner: "Team") sets it, and package.with(owner: nil) removes it. write_package_yml! writes it with no other changes. Any other value raises TypeError.
• metadata.owner is a separate, older convention and isn't read into owner. So owner can be nil for a pack that CodeOwnership.for_package still resolves through metadata.owner.

This bumps the version to 0.28.0, so merging publishes it to RubyGems through cd.yml.

Why owner reads from config instead of being a new prop

My first version added a const :owner prop. Review caught that a package loaded from disk would then hold its owner in two places, and on write the prop would silently override any edit made through config. That's the workaround in use today (package.config['owner'] = ...), and main honours it. So owner is a reader over config['owner'], and with(owner:) edits that key. With a single store, the two can't disagree, and every path that works on main behaves exactly as before.

A value that isn't a String (e.g. owner: 123) reads as nil and is written back unchanged. A typed prop would have raised TypeError inside ParsePackwerk.all on such a file.

with(owner:) edits the copy of config that T::Struct#with returns, rather than passing config: to it. T::Struct#with stringifies every key it's given, so passing config: would have rewritten a nested key such as 1: as '1':.

Package.new has no owner: argument. To build a package with an owner, pass it in config, as callers such as packs already do, or use .with(owner:).

Test plan

• bundle exec rspec: 68 examples, 0 failures. srb tc and rubocop are clean.
• Every new spec that calls owner or with(owner:) fails against main's lib with NoMethodError or ArgumentError.
• Ran the same scripted scenarios against main's lib and this branch's lib, with preserve_key_order both on and off, and with a string owner, owner: 123, and a metadata-only owner. The scenarios were an unchanged round trip, config['owner'] = ..., with(config:) with the owner changed or removed, and with(dependencies: []). The written files are byte-identical in all 30 runs. A broader harness (14 fixtures, 17 existing-usage scenarios, preserve_key_order on and off: 476 runs) is also byte-identical to main, and with(owner:) writes the same bytes as main's config['owner'] = ... workaround in all 140 runs.
• The test suites of downstream rubyatscale gems (packs, visualize_packs, query_packwerk, chatwerk, danger-packwerk, pack_stats, rubocop-packs) pass the same against main's lib and the first version of this change. That version stored owner as a separate prop, and none of these gems passes owner: to Package.new or with, so the reworked version doesn't change what they exercise.
• CI passes.