fix: add support for unlisted pages

[tylerdave]

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[github-actions]

Docs Site preview: https://79c8bb37.rockgarden.pages.dev

[claude]

Both previously flagged bugs are fixed and the implementation is correct.

Previously flagged issue 1 (folder_index.py — subfolder unlisted check): Resolved. Lines 237–241 now check subfolder_index.frontmatter.get("unlisted") before adding a subfolder as a folder child, mirroring the _should_hide guard that was already there.

Previously flagged issue 2 (tree.py — child pages leaking to parent level on unlisted folder break): Resolved cleanly with a for-else construct. The else block only runs when the folder-traversal loop completes without a break, so child pages of an unlisted ancestor are never inserted into current (which would have been the parent scope). The updated test now also asserts "Details" not in labels, covering the exact case that was missing before.