Fix the deadline_reference decorator's no-parentheses form

[ferruzzi]

Found a couple issues with the @deadline_reference decorator while working on my Summit workshop.

Docs implied that you could use @deadline_reference without parentheses, as was intended behavior, but using it that way would cause a TypeError (without a particularly helpful message) later when it was called. This PR fixes support for the non-paren version, adds a clearer error message for a related instantiation issue, and cleans up the documentation a little to improve clarity.

This is an additive fix, anything that was working before should still work unchanged.

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Generated-by: [Kiro-CLI (Claude Opus 5)] following the guidelines

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