feat(deck-builder): add oathbreaker signatures and commander companions

[matthewevans]

No description provided.

[gemini-code-assist]

Code Review

This pull request implements the 'Companion' mechanic for Commander-family formats, including dedicated deck slots, engine-level validation for companion eligibility, and UI support for revealing and declaring companions. It also includes protocol versioning updates, migration logic for existing saved decks, and comprehensive test coverage for these new features. The implementation correctly integrates with the existing engine infrastructure for mana payments and special actions.

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[github-actions]

Parse changes introduced by this PR

Baseline pending for 18e8dd2d044ca4c5f798255d3cb2819e9dc0d478 — this populates once main publishes its coverage snapshot (a few minutes after that commit landed).