rename to OpenSlimedit, rewrite README with full benchmark results

The plugin evolved from hash-based line references to a simpler approach:
compress tool descriptions, compact read output, add line-range edit
expansion. Benchmarks show 21-33% token savings across 4 models vs
baseline, while the original hashline approach increased usage 9-50%.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>