Improve raw-source inspector: file_tree_content should explain directory content shape

[abegong]

Context

file_tree_content currently reports directory content shape through the same generic classes / outliers structure used elsewhere. That makes the evidence hard to read: the user cannot quickly tell which directories contain parseable markdown, which directories have frontmatter, or what metadata/key-set conventions Katalyst found.

For katalyst inspect ., this inspector should contribute to the story of how the directory works, not just list summarizer artifacts.

Goal

Make file_tree_content answer, at a glance:

• Which directories contain markdown content?
• Which directories parse cleanly, partially, or not at all?
• Where does frontmatter appear, and where is it absent?
• What frontmatter key sets are common by directory?
• Which directories look like likely content collections based on content shape?
• Which directories have content that does not fit their surrounding pattern?

Possible shape

Render a directory-oriented content table or grouped summary with concrete paths. Prefer labels like all parse, some parse failures, frontmatter keys: title, status, and no frontmatter over raw feature tokens such as fmkey:title and parsed.

The output should distinguish structural observations from judgment. It can say what was observed and where; it should not recommend a schema or mutation.

Acceptance criteria

• katalyst inspect . --inspector file_tree_content describes content shape by directory in plain language.
• Parse/frontmatter status is visible without requiring the reader to decode feature tokens.
• Common frontmatter key sets are summarized with representative directories or paths.
• Exceptions are grounded in concrete directories/files and are phrased as observations.
• Existing JSON output remains complete and parseable; any schema changes are intentional and covered by tests.
• Snapshot tests cover directories with frontmatter, without frontmatter, and with at least one parse failure or non-markdown region.

[abegong]

Closed by #114. The old file_tree_content surface was replaced with file_content_shape, which profiles an explicit selected file set and reports common structure, variation, parse/read issues, and complete JSON evidence.