feat: add audit-comet-expression Claude Code skill

[andygrove]

Summary

Adds a new Claude Code skill .claude/skills/audit-comet-expression/SKILL.md for auditing existing Comet expression implementations for correctness and test coverage.

The skill (/audit-comet-expression <expression-name>) performs a structured audit:

• Studies the Spark implementation across versions 3.4.3, 3.5.8, and 4.0.1, cloning each tag to /tmp/ and diffing behavioral changes across versions
• Reviews the Comet Scala serde, shims, and Rust/DataFusion implementation for correctness (null handling, type dispatch, ANSI mode, getSupportLevel accuracy)
• Inventories existing Comet tests: SQL file tests and Scala tests
• Produces a coverage gap matrix against Spark's own test suite (nulls, boundary values, NaN, multibyte UTF-8, ANSI mode, all input types, Parquet dictionary encoding, cross-version differences)
• Flags implementation gaps such as missing shims or incorrect Incompatible/Unsupported markings
• Offers to implement the missing tests (preferring the SQL file test framework)

This complements the existing review-comet-pr skill, which focuses on reviewing incoming PRs. The audit skill is for proactively checking the quality of expressions already in the codebase.