docs: Document missing environment variables in README

[echobt]

Summary

This PR adds documentation for environment variables that were implemented in the codebase but were missing from the README.

Problem

The README documented only 4 environment variables (VGREP_HOST, VGREP_PORT, VGREP_MAX_RESULTS, VGREP_CONTENT) while the source code in src/config.rs actually supports additional environment variables for configuration overrides. Users had no way of knowing about these options without reading the source code.

Solution

Updated the Environment Variables section in the README to include all supported environment variables:

• VGREP_CHUNK_SIZE: Characters per chunk when indexing (default: 512)
• VGREP_CHUNK_OVERLAP: Overlap between chunks in characters (default: 64)
• VGREP_MAX_FILE_SIZE: Maximum file size to index in bytes (default: 524288)
• VGREP_WATCH_DEBOUNCE: File watcher debounce delay in milliseconds (default: 500)

The section has also been reformatted as a table for better readability and consistency with other sections of the documentation, along with practical usage examples.

Testing

Verified that all documented environment variables match the implementation in src/config.rs and src/cli/commands.rs.

Related Issue

Fixes PlatformNetwork/bounty-challenge#35