Adds a focused feature file that directly exercises the diff-coverage
gaps reported by the coverage gate across 13 prior attempts:
acms/index.py lines 597-612 (_read_file: stat + size-check + read_text)
context.py lines 111-112 (add_command: traverser invocation)
Three scenarios, each targeting specific line ranges:
1. _read_file with default traverser covers lines 597-598, 611-612
2. _read_file with max_file_size=1 covers lines 602-607
3. add_command with real ChunkedFileTraverser covers lines 111-112
(also transitively covers 597-612 via _read_file)
ISSUES CLOSED: #9982
