fix: standardize release plan state boundary

[TorinAsakura]

Task

• Close [Feature] Standardize release pipeline state boundaries #742

What Changed

• Removed the release ownership metadata surface from Raijin release plans.
• Kept release plans as a narrow pipeline artifact: schema version plus fixed workspace release/decline decisions.
• Kept target versions and version decisions owned by Yarn deferred version files.
• Made release plan creation fail when changed release workspaces do not have Yarn deferred release decisions.
• Kept schema version 2 so legacy plan content cannot be treated as the current format.

Contract

• Yarn owns version decisions and target versions through deferred version files and native version commands.
• Raijin release plans do not store ownership metadata, target versions, or copied version strategies.
• Raijin release plans only preserve the fixed workspace selection needed by later publish stages.
• Follow-up release pipeline tasks must build on Yarn/GitHub-native state instead of adding a parallel Raijin release metadata model.

How To Test

Main Scenario

1. Context: release plan creation after Yarn deferred versioning
   Action: create a release plan through @atls/yarn-plugin-release
   Expected result: the plan contains schema version and release/decline workspace decisions, while target versions remain Yarn-owned state

Additional Scenario

1. Context: release plan parsing
   Action: parse malformed or legacy schemaVersion 1 release plan content
   Expected result: parsing fails instead of accepting ambiguous release state

Proofs

• yarn test unit release-plan --test-reporter=tap
• targeted yarn typecheck for changed release files
• targeted yarn lint for changed release files
• yarn workspace @atls/yarn-plugin-release build
• yarn workspace @atls/yarn-cli build
• cmp -s yarn/cli/dist/yarn.mjs .yarn/releases/yarn.mjs
• yarn raijin:check
• yarn check
• scoped git diff --check excluding generated yarn.mjs bundles

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