Add Idiomatic Rust Review skill + Claude Code plugin manifest

[kzadorozhny]

Summary

This PR adds an installable Claude Code skill that turns the resources collected in this repository into an automated Rust code reviewer, plus the marketplace manifests needed so anyone can install it with a single command.

After this lands, users will be able to run /plugin marketplace add mre/idiomatic-rust followed by /plugin install idiomatic-rust-review@idiomatic-rust from Claude Code, and the skill will activate automatically whenever they paste Rust code or ask for a Rust review.

What's included

• skills/idiomatic-rust-review/SKILL.md — a peer-reviewed ruleset (12 top-tier rules with severity tags MUST / SHOULD / MAY, a reviewer process checklist, a "what not to flag" list, and a structured output format) synthesized from this repository's corpus, the Rust API Guidelines, Canonical's Rust Best Practices, Elements of Rust, and Clippy.
• skills/idiomatic-rust-review/references/ — 12 category files (error handling, types & conversions, ownership & lifetimes, iterators, API design, async & concurrency, pattern matching, documentation & naming, unsafe & performance, anti-patterns, a flat rule index, and a worked before/after example) that the reviewer loads on demand, with each rule citing its source from this corpus.
• .claude-plugin/marketplace.json + .claude-plugin/plugin.json — minimal Claude Code plugin manifests so the repo doubles as a plugin marketplace. The skill is sourced at ./, so the existing skills/ directory is auto-discovered.
• resources.json + render/templates/README.md — registers the skill as a project entry and adds a "🤖 Use as a Claude Code Skill" section to the rendered README with the install commands. README.md is regenerated via make render.

Design notes

The skill ruleset is opinionated but deliberately conservative — it scopes which rules are MUST vs SHOULD vs MAY, tells reviewers to pick the 5–10 highest-value findings rather than nitpicking, and includes an explicit "what NOT to flag" list. Every rule cites the source resource it came from, most of which are already in resources.json, so the review output stays traceable to this corpus.

Plugin manifests are intentionally minimal (just name, description, version, owner). The plugin's source is ./ so adding more skills to this repo later is a one-line change to marketplace.json.

Test plan

• make render runs cleanly and regenerates README.md with the new section and resource entry
• In Claude Code: /plugin marketplace add mre/idiomatic-rust resolves the manifest
• /plugin install idiomatic-rust-review@idiomatic-rust installs the skill from the marketplace
• After install, pasting a .rs snippet into Claude Code activates the skill and produces a review in the documented output format
• /plugin uninstall idiomatic-rust-review@idiomatic-rust cleanly removes it
• Existing CI (the Check Links workflow) still passes — no new external URLs were added that weren't already in the corpus