feat: centralize state file location and fix clear command behavior

Summary

This MR centralizes CLI state management by moving all state files to a dedicated directory in the user's home folder: ~/.corpus-cli/. Previously, state files were stored in the project's working directory, which caused inconsistent behavior when running the CLI from different locations.

Changes Made

• Centralized State Paths — All state files now live in ~/.corpus-cli/:
  • corpus_client_state.json (auth session)
  • extracted_text_state.json (extracted text upload progress)
  • uploaded_extracted_text.log (legacy upload log)
  • upload_state.json, upload_log.txt, failed_uploads.txt (record uploads)
• Updated modules: cli.py, upload.py, extracted_text_upload.py
• Fixed clear command — Now removes all state files including uploaded_extracted_text.log, using module-level constants instead of hardcoded filenames

Bug Fixes

• Fixed clear command not removing uploaded_extracted_text.log

🧪 Testing

• ✔ State files created in ~/.corpus-cli/
• ✔ CLI works from any directory
• ✔ clear removes all state files
• ✔ No state files created in project directory

🔗 Related Issue

Closes #15 (closed)