v1.12.2: preflight gates SKILL.md frontmatter

Added

• scripts/preflight.py checks every SKILL.md frontmatter: the block must be closed, carry name and description, and no unquoted value may contain : or #. That last rule is the v1.12.0 handoff bug, which the gate passed. It is a targeted check, not a full YAML parse, so the script stays stdlib-only.

Full Changelog: v1.12.1...v1.12.2