docs: use session commands for reading mutation results

[dkubb]

Summary

• Replace terminal output parsing with mutant's built-in session commands (session subjects, session show) for reviewing results in SKILL.md.
• Add session commands to Reading Output section as primary method.
• Replace sed pipeline in Prepare Legacy Project with mutant session subjects.

Test plan

• Verify SKILL.md renders correctly on GitHub.
• Confirm session commands match documented behavior.