Add random Rig sample report workflow

[Copilot]

Adds a daily agentic workflow that randomly selects five Rig samples, executes each through a delegated subagent, and reports outputs with structured runtime logs.

• Sample execution

  • Selects exactly five samples per run.
  • Extracts complete rig fences and executes them through the Rig launcher.
  • Captures status, output, and bounded rig.* logs.

• Reporting

  • Publishes one issue with per-sample results and grouped runtime logs.
  • Avoids duplicate, high-volume prompt and response logging.

• Workflow configuration

  • Enables the Copilot SDK engine and pinned Rig skill.
  • Adds Agentic Workflow setup and compilation guidance to SKILL.md.

engine:
  id: copilot
  copilot-sdk: true
skills:
  - githubnext/rig/skills/rig/SKILL.md@<full-commit-sha>