Auto-detect input format from file extension

[vmvarela]

Description

When a file argument has a recognizable extension (.csv, .tsv, .json, .ndjson, .xml), auto-set the input format. -I still works as an override.

Examples

# No -I needed — extension tells sql-pipe it's JSON
sql-pipe data.json 'SELECT * FROM t WHERE score > 80'

# Override when extension is wrong
sql-pipe -I tsv data.txt 'SELECT * FROM t'

Acceptance Criteria

• File extensions .csv, .tsv, .json, .ndjson, .xml auto-set input format
• -I flag still works as explicit override
• Stdin input still defaults to CSV (no filename to inspect)
• Ambiguous extensions (.txt, .dat) default to CSV
• All existing tests pass
• New tests cover auto-detection and override behavior

Notes

• Implementation: ~30 lines
• Map file extension to InputFormat enum in argument parser
• Only applies to file arguments, not stdin
• Zero new flags, actually removes a flag from common invocations