feat: add constants syntax

[hntrl]

chatty-g cooked this one up. basic idea is that we specify what the "latest and greatest" model/model strings is this constants map once and use it throughout docs:

Summary

• Introduce a reusable CONSTANTS_MAP registry so docs can reference shared strings with the $[...] token syntax.
• Add a replace_constants preprocessor that swaps tokens for their mapped values, leaves unknown constants untouched, and supports escaping.
• Wire the new substitution step into preprocess_markdown and cover the behaviour with focused unit tests, including a smoke test of the full markdown pipeline.

[github-actions]

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