feat: add Rust language support to codebase-readiness

[acostanzo]

Summary

• Add first-class Rust language support to the codebase-readiness assessment plugin, bringing supported languages from 7 to 8
• Create rust.md language reference covering all 8 assessment dimensions with Rust-specific signals
• Update recon.sh to detect Cargo.toml, count .rs source files, detect Rust integration tests in tests/, and find clippy.toml/rustfmt.toml linter configs
• Add Rust to README example language list
• Bump plugin version from 1.5.0 → 1.6.0

Details

Rust was already referenced in SKILL.md's weight tables as a statically-typed language but had no language reference file and wasn't detected by the reconnaissance script. This PR closes that gap.

The rust.md reference follows the exact section structure of go.md (both statically-typed with similar philosophies) and covers:

• Type Safety — unsafe blocks, unwrap()/expect() frequency, transmute/raw pointers, clippy::unwrap_used lint detection, newtypes, #[must_use], From/Into impls, thiserror/snafu/anyhow/color-eyre error handling
• Test Foundation — #[cfg(test)] in-file tests, tests/ integration tests, doc tests (/// and //!), insta snapshot testing, proptest/criterion/mockall/rstest
• Feedback Loops — cargo clippy, cargo check, cargo-watch/bacon, cargo-audit, cargo-deny, cargo-nextest
• Code Clarity — snake_case/PascalCase conventions, 2015 vs 2018+ module system
• Consistency — rustfmt.toml, clippy.toml, rust-toolchain.toml, deny.toml, #[deny(warnings)] anti-pattern detection, missing_docs enforcement, observability crate detection
• Architecture — standard layout, workspace structure with dependency inheritance
• Change Safety — #[cfg(feature)], cargo-audit/cargo-deny, migration tools, MSRV policy, edition detection, toolchain pinning

All evidence commands use grep -rE (ERE) for macOS/Linux portability and follow the unanchored grep -v target | grep -v .git convention used by all other language files.

Test plan

• quick_validate.py passes: Skill is valid!
• rust.md section headers match go.md exactly (zero diff)
• Version bumps match in both plugin.json and marketplace.json
• No BRE \| alternation patterns (macOS-safe)
• No anchored grep filters (matches convention of other language files)
• No duplicate evidence commands across sections
• Run bash scripts/recon.sh in a Rust project to confirm detection

[dgalarza]

Thanks for adding!