feat: add --silent / -s flag to suppress row count output

[vmvarela]

Summary

• Adds --silent / -s flag that suppresses the rows-processed count from stderr, producing clean stdout-only output suitable for piping or scripting
• --silent and --verbose are mutually exclusive; passing both returns an error with a descriptive message
• Help text documents -s, --silent alongside -v, --verbose

Changes

• src/main.zig: new SilentVerboseConflict error, silent: bool field in ParsedArgs, parsing of --silent/-s, mutual exclusion with --verbose/-v, suppressed stderr row-count in run()
• build.zig: 4 new integration tests (71–74) covering silent mode, -s alias, and conflict with --verbose/-v

Acceptance Criteria

• --silent and -s are accepted as equivalent flags
• When --silent is set, the rows-processed count is not printed to stderr
• --silent and --verbose are mutually exclusive; passing both returns an error
• Help text (--help) documents --silent / -s alongside --verbose / -v
• Tests cover silent mode output and the mutually-exclusive conflict

Closes #114