Update plugin creation agent guidance

[eric-tramel]

What

Updates .claude/commands/create-plugin.md so the plugin creation command reflects the current repository structure, ddp scaffold output, Makefile target names, and Zensical documentation workflow.

Adds .codex/skills/data-designer-plugin-authoring/SKILL.md as a Codex-native project skill for the same Data Designer plugin authoring flow.

Why

The existing Claude command had drifted from the repo: it referenced stale paths and targets such as docs/adding-a-plugin.md and make catalog, and it did not explain how to prepare per-plugin docs for the generated Zensical site.

Usage

Claude command example:

/create-plugin word-count - counts words in text columns

Codex skill usage: ask Codex to create, update, or document a Data Designer plugin in this repo. The skill points Codex to the current repo guidance, scaffold flow, implementation conventions, validation targets, and per-plugin Zensical docs format.

Per-plugin docs should live under plugins/data-designer-<slug>/docs/, typically with index.md and optionally usage.md, then be regenerated and checked with:

make plugin-docs
make docs

How

The Claude command was rewritten around the current AGENTS.md, docs/authoring.md, docs/workflow.md, devtools/ddp scaffold, plugin_docs.py, and Makefile targets. The new Codex skill mirrors the same guidance in project-skill format under .codex/skills/.

Validation

• make sync
• make format
• make lint
• make test
• make validate
• make check
• make docs
• git diff --cached --check

Additional Codex skill smoke test:

• Ran a fresh codex exec session in /tmp/ddp-codex-skill-test.hgHHHM/DataDesignerPlugins against the current PR branch.
• Asked it to create a vowel-count plugin with config, implementation, tests, and per-plugin docs.
• The agent used the scaffolded package path plugins/data-designer-vowel-count/, consistent with uv run ddp new vowel-count, then edited the generated files rather than hand-assembling the initial skeleton.
• The generated plugin passed make format, make test-plugin PLUGIN=data-designer-vowel-count with 17 tests, make lint, make validate, make check, make docs, and full make test across the template and new plugin.