Add --dictionary CLI argument to ingest word list from file

[Copilot]

This PR adds support for a --dictionary CLI argument that allows users to provide a newline-separated text file containing search needles, as requested in the issue.

Changes Made

• Added clap dependency with derive features for argument parsing
• Introduced a new Args struct using clap's derive API to handle CLI arguments
• Modified gather_needles() function to accept both dictionary files and CLI arguments
• Maintained full backward compatibility with existing CLI usage
• Added proper error handling for file reading and validation

Usage Examples

Using a dictionary file:

cargo run -- --dictionary words.txt

Using CLI arguments (existing behavior):

cargo run -- hello world test

Combining both approaches:

cargo run -- --dictionary words.txt extra additional

Implementation Details

The dictionary file is processed with the same preprocessing steps as CLI arguments:

• Converts all words to lowercase
• Removes whitespace from words
• Deduplicates the final set of needles
• Filters out empty strings

Error handling includes:

• Clear error messages for missing dictionary files
• Validation that at least one needle is provided (either via file or CLI)
• Graceful handling of empty dictionary files

The implementation preserves all existing functionality while adding the new dictionary mode as requested.

Fixes #4.

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