feat: add --skip-subtrees-wo-markdown option to prune non-documentation subtrees

[geopanther]

Summary

Adds a new --skip-subtrees-wo-markdown CLI option that, when uploading a directory, skips any subtree that doesn't contain markdown files. This avoids creating empty Confluence pages for non-documentation directories like images/, data/, or config/.

Motivation

When using md2cf to upload a project folder, directories containing only non-markdown assets (images, configs, data files) are traversed and produce empty folder pages in Confluence. There's currently no way to prune these automatically — --skip-empty and --collapse-empty only handle folders without direct children, not entire subtrees. This option provides a clean way to focus uploads on documentation content only.

Changes

Core logic (md2cf/document.py)

• New _subtree_has_markdown(path, git_repo) helper that recursively checks whether a directory tree contains any .md files, respecting .gitignore rules
• New skip_subtrees_wo_markdown parameter on get_pages_from_directory() that prunes directories[:] in-place during os.walk, preventing descent into subtrees without markdown

CLI wiring (md2cf/__main__.py)

• New --skip-subtrees-wo-markdown flag in the directory arguments group
• Passed through collect_pages_to_upload() → get_pages_from_directory()

Tests (test_package/unit/test_document.py)

Five new test cases using pyfakefs:

Test	Scenario
test_..._skip_subtrees_wo_markdown	Basic pruning: sibling dirs without .md files are removed
test_..._nested	Deeply nested markdown is preserved; sibling non-md subtrees are pruned
test_..._root_has_md	Root-level .md files kept; child subtrees without markdown pruned
test_..._all_empty	No subtree has markdown → empty result
test_..._disabled	Flag off vs. on comparison — non-md subtrees only pruned when enabled

Documentation (README.md)

• New "Skipping subtrees without markdown" section under "Directory arguments" with usage description and collapsible example

Design decisions

• Pruning happens at the os.walk level by modifying directories[:] in-place, which prevents os.walk from descending into pruned subtrees at all — no wasted I/O
• Gitignore is respected in the subtree check, consistent with existing behavior
• Off by default — no change to existing behavior unless explicitly opted in