Add /mine-tasks skill for automatic benchmark task discovery from codebases

Analyzes user-provided repos (GitHub URL or local path) to propose benchmark
tasks: mines closed issues/PRs for SDLC tasks, analyzes codebase structure
for org-scale tasks. Outputs proposals compatible with /scaffold-task.

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

Author: sjarmak