Add --validate option to show command

[contributor]

This adds --validate option to show command.

Motivation:
Often agents create chapters file with missing hunks. show command catches that, but it is too late (the server and the browser is already started).

--validate option gives ability to validate chapters (or instruct agents to validate first) without starting server/browser

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Summary by cubic

Adds a --validate option to the show command so you can check chapters for missing hunks without starting the server or opening the browser. Running show as before still serves and opens; passing --validate runs the same chapter-building step and stops after validation.

^{Written for commit 287b91b. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}