Add README for plain.skills

Author: davegaeddert

plain/plain/skills/README.md

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+# plain.skills
+
+**Agent skills for working with Plain projects.**
+
+These skills provide context and workflows for common tasks when using [Claude Code](https://docs.anthropic.com/en/docs/claude-code) or [Codex](https://codex.openai.com/).
+
+## Available skills
+
+| Skill              | Description                                                    |
+| ------------------ | -------------------------------------------------------------- |
+| `plain-docs`       | Retrieves detailed documentation for Plain packages            |
+| `plain-install`    | Installs Plain packages and guides through setup steps         |
+| `plain-upgrade`    | Upgrades Plain packages and applies required migration changes |
+| `plain-shell`      | Runs Python with Plain configured and database access          |
+| `plain-request`    | Makes test HTTP requests against the development database      |
+| `plain-principles` | Provides Plain framework context and coding conventions        |
+
+## Installation
+
+To install skills to your project's `.claude/` or `.codex/` directory:
+
+```bash
+uv run plain skills --install
+```
+
+This copies the skill definitions so your agent can use them. Run it again after upgrading Plain to get updated skills.