Positional file arguments and multi-file joins

[vmvarela]

Description

Accept files as positional arguments. Each file becomes a table named after its basename (sans extension). Stdin remains t.

Examples

# Single file — no more cat
sql-pipe orders.csv 'SELECT * FROM orders WHERE amount > 100'

# Multi-file join — the #1 reason people reach for DuckDB
sql-pipe orders.csv customers.csv \
  'SELECT c.name, SUM(o.amount) FROM orders o JOIN customers c ON o.cust_id = c.id GROUP BY c.name'

# Stdin still works, still called t
cat data.csv | sql-pipe 'SELECT * FROM t'

# Mix stdin + files
cat events.csv | sql-pipe users.csv 'SELECT * FROM t JOIN users ON t.uid = users.id'

Acceptance Criteria

• Files passed as positional arguments are loaded as tables
• Table names are derived from filenames (basename sans extension)
• Stdin input is still available as table t
• Multi-file joins work correctly
• Input format is auto-detected from file extension (see related issue)
• File-vs-query disambiguation works (heuristic: if arg is a readable file, treat as input; query is last non-file argument)
• -- separator works for disambiguation if needed
• All existing tests pass
• New tests cover single-file, multi-file, stdin+file, and edge cases

Notes

• This subsumes the "federated query" part of Database export and federated query support #70
• Implementation: ~150 lines of Zig
• Update args.zig to distinguish file paths from query string
• Table naming: handle invalid SQL identifiers in filenames (quote or sanitize)

[vmvarela]

Implemented in #162.