Add --schema flag to show inferred CREATE TABLE DDL

[vmvarela]

Description

Add a --schema flag that prints the CREATE TABLE statement that sql-pipe inferred from the input data, without executing a query.

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

# With file arguments:
sql-pipe orders.csv customers.csv --schema
# CREATE TABLE "orders" (...)
# CREATE TABLE "customers" (...)

Motivation

Users frequently need to understand what sql-pipe "sees" from their data. Currently --columns --verbose shows types, but DDL output is more precise and allows users to understand affinity, see the exact schema, and even copy it into a persistent database. Especially useful for debugging type inference issues (e.g., why a column was detected as TEXT instead of INTEGER).

Acceptance Criteria

• --schema flag is parsed in args.zig
• After loading tables, query SELECT sql FROM sqlite_master WHERE type='table' and print each row
• Works with multiple files (shows schema for each table)
• --schema is mutually exclusive with --columns, --validate, --sample, --stats, --explain, and a query argument
• Output goes to stdout (can be piped to a file)
• Integration tests cover the new mode
• Help text updated

Implementation Notes

• Add SchemaArgs to src/args.zig following the pattern of ColumnsArgs/ValidateArgs
• The existing getTableColumns function in src/sqlite.zig (line 277) already knows how to query PRAGMA table_info
• The DDL query is trivially added: SELECT sql FROM sqlite_master WHERE type='table' AND name='<table>'