fix(diff): extract symbols from new working-tree files

New files (uncommitted) produce a plain InsertOp with no symbol detail because the object store doesn't have their content yet. Read bytes directly from disk via repo_root / path when the object store misses, then extract symbols for .md (ATX headings) and .py (def / class).

Return a PatchOp with InsertOp child ops so _print_structured_delta calls _print_child_ops and displays section/function names inline under the 'A filename' line — matching what the TestDiffWorkingTreeSymbols tests assert.