init recipe doesn't detect Ruby (or other runtime version pinning)

## Problem

The init Goose recipe at `src/templates/init_rule.md` enumerates manifest files to scan (`package.json, Cargo.toml, go.mod, pyproject.toml, Makefile, Justfile, Taskfile, build.gradle, pom.xml, composer.json, mix.exs`) but omits Ruby entirely — no `Gemfile`, no `gems.rb`.

It also has no general step for detecting runtime version pinning files (`.ruby-version`, `.nvmrc`, `.python-version`, `.tool-versions`, etc.).

The result: when `ai-rules init` runs in a Ruby repo, the generated `example.md` never tells the agent which Ruby to use. Coding agents default to whatever `/usr/bin/ruby` resolves to (often macOS system Ruby 2.6.x), which:

- fails to parse modern Ruby syntax (e.g. comments between fluent dots, added in Ruby 2.7)
- can't resolve gems pinned to a newer ABI
- skips pre-commit/pre-push hooks that depend on a project-pinned Ruby

This affects every Ruby service that has run \`ai-rules init\`. The same gap exists for any repo pinning Node/Python/etc. via \`.nvmrc\` / \`.python-version\` if the surrounding manifest doesn't carry version info.

## Proposed fix

Two surgical edits to \`src/templates/init_rule.md\`:

1. Add \`Gemfile, gems.rb\` to the manifest list in Step 2, and instruct the recipe to read the \`ruby ""\` directive.

2. Add a new **Step 3: "Detect runtime version pinning"** that finds \`.ruby-version\`, \`.nvmrc\`, \`.node-version\`, \`.python-version\`, \`.tool-versions\`, \`rust-toolchain.toml\`, \`mise.toml\`, cross-references them against manifest declarations, and — if found — requires the generated rule file to include a **"Runtime / Toolchain"** section with the exact activation command (e.g. \`rvm use \$(cat .ruby-version)\`).

The new section maps each pinning file to its common version managers, defaulting to whichever the user has on \`\$PATH\` (rvm for Ruby, nvm for Node, pyenv for Python).

Draft PR to follow.

Start with src/templates/init_rule.md, especially the manifest enumeration in Step 2. Add the Ruby manifests and runtime-version detection described in the issue, then verify that generated example.md includes a Runtime / Toolchain section with the appropriate activation command when pinning files are present.

Written by the indexing model from the issue text.