Take the design-tree skill from Design-skill as a submodule

[mbbill]

The design-tree skill moves out of Whitefoot into Design-skill (Ming-Research/Design-skill#1). Snowghost now takes it from there as a submodule, and the repository's name becomes Snowghost-wf.

Status

• Skill as a submodule: done. design/skill is a submodule of Design-skill, pinned at 1647bdc, the head of Move the design-tree skill here from Whitefoot Design-skill#1. Before this PR merges, the pin moves to that PR's commit on Design-skill's main (rule 4).
• Project name: done. The README and AGENTS.md name it Snowghost-wf, with "Snowghost for short"; other prose keeps Snowghost.

Changes

• design/skill/: the three copied files (SKILL.md, lint.py, test_lint.py) are replaced by the submodule. Its SKILL.md and lint.py keep the same content apart from one added sentence in Lint, and test_lint.py changes only the paths it uses to find the scripts (see Design-skill#1).
• .github/design-review-base.sh is deleted. It moved into the skill as review-base.sh, and both workflows call design/skill/review-base.sh.
• design-readiness.yml runs git submodule update --init design/skill and skips the Whitefoot submodule, which this 2-minute job does not need. check.yml already checks out submodules.
• The paths, the .claude/skills and .agents/skills links and the Makefile targets are unchanged.
• AGENTS.md: names the new submodule, extends rule 4 and "exact revision" to cover both pins, says that the skill is changed only in Design-skill, and says the checks need both submodules. The PR template's pin note follows rule 4.

Decision cards

None. No design-tree node changes, so no log entry is needed. The rule changes in AGENTS.md follow the owner's direction in conversation: split the skill out, use it as a submodule, and name the projects with -wf.

Validation

• CI on this branch (make check, which runs design-lint and with it test_lint.py from the submodule). Nothing was compiled or tested locally.
• design-readiness runs only on ready PRs and on main, so its new submodule step is not exercised until then.

Found along the way

• The README linked Whitefoot at mbbill/Whitefoot. Fixed to Ming-Research/Whitefoot.
• The log and research records cite older PRs as mbbill/Snowghost#NN. These are approval records and historical citations, so they are left unchanged; GitHub redirects the old name.
• On this machine the whitefoot/ submodule has never been initialized in the main checkout or in this worktree. With the skill now a submodule too, an agent in a fresh worktree sees no skill until git submodule update --init runs.

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