Remove or justify unreachable code

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From PR #1 review nits.

APSError.unknownKey can never be thrown (ArgumentParser rejects unknown keys first; the spec's own Error Cases section says so) and JSONCoding.decode is only exercised by tests. Both are documented spec exports, so removal means spec churn; keeping them means the contract documents dead code.

Acceptance criteria

• Either delete both (and their spec rows/tests) or add a short documented rationale for keeping them
• specsync check stays green

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Claimed by claude-code. Working on branch claude-code/issue-15-remove-unreachable-code

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PR opened: #28

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Claimed by agent:cursor. Working branch: cursor/issue-15-remove-unreachable-code (rebasing PR #28 onto current main with a fresh SpecSync change after docs #41).