Add s3_upload tool for S3-compatible file uploads

[frival]

Summary

• Adds s3_upload, a Python 3 tool (zero external dependencies) for uploading files to S3-compatible storage services (AWS S3, MinIO, Ceph, etc.)
• Supports two mutually exclusive auth methods: username/password (AWS Signature V4) and access token (Bearer)
• Credentials can be passed via CLI args or environment variables (S3_USERNAME, S3_PASSWORD, S3_ACCESS_TOKEN)
• Outputs the uploaded file URL to stdout and logs timestamped entries to a configurable log file (/tmp/s3_upload.log by default) for use by subsequent tools
• Updates README.md with full documentation and error_codes comment

Test plan

• Verify --help displays usage information
• Verify mutual exclusivity of --username and --access-token (CLI and env var)
• Verify --password is required when --username is provided
• Verify file-not-found error handling
• Test upload to an S3-compatible endpoint (e.g. MinIO) with username/password
• Test upload to an S3-compatible endpoint with access token
• Verify uploaded file URL is printed to stdout
• Verify log file is written with timestamp and URL
• Verify environment variable fallback works when CLI args are omitted

🤖 Generated with Claude Code