feat: add SQL language support to parser

Adds .sql file parsing via tree-sitter-sql (already in language pack):
- CREATE TABLE/VIEW → Class nodes with extra["sql_kind"]
- CREATE FUNCTION → Function nodes with extra["sql_kind"]="function"
- CREATE PROCEDURE → regex fallback (grammar emits ERROR for this node)
- FROM/JOIN table references → IMPORTS_FROM edges for impact analysis

Includes fixture (tests/fixtures/sample.sql) and 8 TestSQLParsing tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>