fix(build-cli): preserve valid admonition line breaks

[shlevari]

Description

Fix stripSoftBreaks so it preserves the structural newline after a valid GitHub admonition marker without introducing newlines for marker-like text elsewhere.

Previously, soft breaks were removed before admonition markers were examined, which erased whether [!NOTE] appeared on its own source line. The transform then reconstructed line breaks from the flattened text, incorrectly treating same-line content as an admonition and inserting breaks before marker-like text in blockquote bodies.

The new test suite covers all supported admonition types, trailing whitespace, block content, body normalization, same-line markers, later markers, and unsupported marker forms.

Reviewer Guidance

The review process is outlined in the pull request guidelines.

The key behavior to review is that admonition recognition now uses the original Markdown AST structure before soft-break normalization.

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