Add stdout output and quiet mode support

[williamjameshandley]

Summary

This PR addresses issue #115 by implementing UNIX-style stdout output and quiet mode functionality:

• Stdout output: Use - as the output filename to write to stdout following UNIX conventions
• Quiet mode: Add --quiet/-q flag to suppress progress and success messages
• Smart clipboard behavior: Automatically disable clipboard when outputting to stdout (unless explicitly enabled)

Changes Made

• Added --quiet/-q CLI flag to suppress progress messages
• Modified write_to_file() function to support stdout output when filename is "-"
• Updated spinner and message display logic to respect quiet mode
• Automatically disable clipboard copying when writing to stdout

Test Plan

• Verify --quiet flag appears in help output
• Test stdout output with -O - (writes to stdout, no clipboard)
• Test quiet mode suppresses progress messages
• Test normal file output still works with success messages
• Test quiet mode with file output (suppresses file write message)
• Verify clipboard behavior is disabled when outputting to stdout

Examples

# Output to stdout with quiet mode (ideal for vim workflows)
code2prompt /path/to/code --quiet --output-file -

# Output to stdout with progress messages
code2prompt /path/to/code --output-file -

# Quiet mode with file output
code2prompt /path/to/code --quiet --output-file output.txt

Fixes #115

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