Add PRQL query language support

[ndenev]

Summary

• Add PRQL (Pipelined Relational Query Language) as an alternative to SQL
• Auto-detection based on syntax - no mode switching needed
• PRQL queries are compiled to SQL transparently before execution

Example

from pods
filter namespace == "kube-system"
select {name, namespace, created}
sort created
take 10

Equivalent SQL:

SELECT name, namespace, created FROM pods 
WHERE namespace = 'kube-system' 
ORDER BY created LIMIT 10

Implementation

• Add prqlc = "0.13" dependency
• New src/datafusion_integration/prql.rs with detection and compilation
• Integrate into preprocess_sql() which now returns Result<String>
• Reorder execute_sql: preprocess before validation (PRQL needs compilation first)

Test plan

• cargo build passes
• cargo test passes (165 tests)
• cargo clippy clean
• Manual testing: k8sql -q "from pods | take 3" works
• Manual testing: SQL queries still work unchanged

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.