fix: rebuild selected_benchmark_tasks.json to match 275 canonical tasks

Was stale (472 tasks, 111 missing canonical, 308 non-canonical extras).
Now exactly 275 tasks matching unified_benchmark_manifest.json with
task_dir and benchmark fields required by run scripts.

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

Author: sjarmak