feat: CI validation for skill code blocks

## Context

Skills in `.claude/skills/` contain Rust code blocks that serve as reference patterns for AI agents. These code blocks can go stale when the SDK updates (e.g., API changes, renamed types, new required parameters).

Raised in PR #18 review as a follow-up for the maintenance strategy.

## Proposal

Add a CI step that:
1. Extracts Rust code blocks from all `SKILL.md` files
2. Attempts to compile them against the current SDK version
3. Fails the CI run if any code block no longer compiles

This catches skill staleness early rather than waiting for an agent to use a broken pattern.