fix: scope contributor skills to the SkipHow repository

[mzored]

Summary

• streamline the contributor-only Dogfood skill from 1,266 to 736 words around evidence reconstruction, causal judgment, and the smallest justified package change
• add the same project-scoped Dogfood skill for Codex and keep the Codex and Claude copies aligned
• disable the shipped SkipHow plugin inside its own repository while leaving it available elsewhere
• keep project-only .agents/skills out of the Codex marketplace payload

Verification

• Added a host-packaging regression assertion.
• Ran python scripts/check.py.
• Ran python scripts/check.py --pytest tests/test_hosts.py (23 passed).
• Ran git diff --check.
• Ran python scripts/check_hosts.py.

Codex and Claude package validation passed. Claude isolated install passed. Codex isolated install is UNVERIFIED because the host policy forbids a temporary local marketplace; all three configured Codex profiles were checked directly and report dogfood=1, skiphow=0 in this repository and dogfood=0, skiphow=1 outside it.

Notes

This changes contributor tooling and host packaging only. plugins/skiphow/ is unchanged, so no package version bump is required. An independent subagent review found no qualifying defects before commit.