fix(yarn-plugin-library): clean private package publish metadata

[TorinAsakura]

Task

• Close [Package] Clean private package publish metadata contract #707

How to verify

Before fix

1. Context: private Raijin Yarn workspaces used package manifests as both workspace runtime manifests and pack-time manifests.
   Action: Inspect private yarn/*/package.json manifests or pack a private Yarn plugin.
   Expected result: Before fix, private internal packages carried publishConfig, mixing registry publish metadata with pack metadata.

After fix

1. Context: private Raijin Yarn workspace manifests.
   Action: Inspect private yarn/*/package.json manifests.
   Expected result: Private workspaces keep private: true, do not define publishConfig, and expose pack-only metadata through the internal raijin.pack contract consumed by @atls/yarn-plugin-library.

2. Context: packed private Yarn plugin manifests.
   Action: Pack @atls/yarn-plugin-checks and @atls/yarn-plugin-library, then inspect package/package.json inside each archive.
   Expected result: Packed manifests contain dist-oriented main, types, and exports, and contain neither publishConfig nor the internal raijin field.

Proofs

• yarn test unit before-workspace-packing --test-reporter=tap passed.
• yarn workspace @atls/yarn-plugin-checks pack --out /tmp/raijin-yarn-plugin-checks.tgz proof passed: packed manifest has main: dist/index.js, types: dist/index.d.ts, exports["."].import: ./dist/index.js, no publishConfig, no raijin.
• yarn workspace @atls/yarn-plugin-library pack --out /tmp/raijin-yarn-plugin-library.tgz proof passed with the same packed manifest contract.
• yarn workspace @atls/yarn-plugin-library build passed.
• yarn workspace @atls/yarn-cli build passed.
• cmp -s yarn/cli/dist/yarn.mjs .yarn/releases/yarn.mjs passed.
• git diff --check -- ':!yarn/cli/dist/yarn.mjs' ':!.yarn/releases/yarn.mjs' passed.
• yarn check exited 0. Existing unrelated lint warning remains in config/prettier/src/index.ts.
• yarn raijin:check exited 0.