feat: add --schema flag to print inferred CREATE TABLE DDL

[vmvarela]

Summary

Adds a --schema flag that loads input data into SQLite and prints the inferred CREATE TABLE DDL to stdout without executing a query. Closes #176.

Changes

• src/args.zig — Added SchemaArgs struct, SchemaWithFlags error variant, --schema flag parsing, mutual exclusivity checks, and help text.
• src/modes/schema.zig (new) — runSchema() follows stats.zig pattern: opens in-memory DB, loads tables, queries sqlite_master, prints DDL.
• src/main.zig — Import, dispatch, and error handler for schema mode.
• build.zig — 16 integration tests covering stdin, files, types, --no-type-inference, TSV, JSON, empty input, and all mutual exclusivity combos.
• README.md — Added --schema row to the flags table.

Usage

cat data.csv | sql-pipe --schema
# CREATE TABLE "t" ("name" TEXT, "age" INTEGER);

sql-pipe orders.csv customers.csv --schema
# CREATE TABLE "orders" (...);
# 
# CREATE TABLE "customers" (...);

Verification

• zig build test — 264 steps pass
• zig build unit-test — pass
• ziglint — no regressions