v0.16.0 — extension-scoped tool exposure

tool_specs(extension="xlsx") and doc_guide(fmt=...) return only the verbs, topics, and description detail relevant to a file's format — backed by a new tool_matrix capability table (SSOT). Opt-in and fully backward-compatible: the unscoped default is unchanged. arrange_doc is hidden for .docx; set_doc_text/arrange_doc/build_doc descriptions specialize per format. 900 tests pass.