Improved error messages and diagnostics

[vmvarela]

Description

Improve error messages for common user mistakes so sql-pipe fails gracefully with actionable guidance instead of cryptic SQLite errors.

Acceptance Criteria

• Invalid SQL syntax shows a clear message with the error position
• Referencing a non-existent column suggests similar column names (typo detection)
• Empty stdin (no data piped) produces a helpful message instead of silent failure
• Malformed CSV (unclosed quotes, inconsistent columns) reports the problematic line number
• All error messages go to stderr, not stdout
• Exit codes follow the structured scheme from PR feat: structured exit codes, --help and --version flags #17

Notes

• The structured exit codes were added in Sprint 1 (PR feat: structured exit codes, --help and --version flags #17)
• Focus on the most common failure modes users will encounter
• This improves the overall developer experience beyond just features