Add Ruler for managing AI agent instructions

[audreyfeldroy]

Plan

Add Ruler to the outer repo so we can write agent instructions once in .ruler/ and have it generate the tool-specific files (CLAUDE.md, AGENTS.md, .cursorrules, etc.) automatically.

I use Claude Code currently but I switch agentic AI tools regularly. Ruler solves the "one source of truth" problem for agent instructions across tools.

We'll try it on the outer repo first. If it works well, we may add it to the generated project as an optional feature later.

Dependency group

tools (alongside rust-just).