fix(skills): diff installed skills against bundled template by default

[solomonneas]

Summary

• Default brigade skills diff to the bundled Brigade package template when one exists, so stale .brigade/skills/registry/ copies cannot mask drift from newer bundled skills.
• Add --against registry to keep the previous registry baseline when that comparison is intentional.
• JSON output now includes against and baseline_skill for easier debugging.

Closes #484.

Test plan

• pytest tests/test_skills_cmd.py tests/test_skill_templates_metadata.py
• ./scripts/verify (4191 passed, 82.60% coverage)
• Stale registry install + default diff reports changed: true
• Fresh bundled install + default diff reports changed: false
• --against registry reports clean when installed matches registry

Made with Cursor

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