feat: add filesystem demo with file upload and streaming

- Add example demonstrating file upload to Letta data sources
- Include text file upload from disk and string creation
- Add PDF download and upload functionality
- Implement streaming agent responses with interactive chat
- Show how to attach data sources to agents

examples/files/README.md

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+# Letta Files and Streaming Demo
+
+This demo shows how to work with Letta's file upload and streaming capabilities.
+
+## Features
+
+- Upload files from disk to a Letta data source
+- Create files from strings and upload them
+- Download and upload PDF files
+- Create an agent and attach data sources
+- Stream agent responses in real-time
+- Interactive chat with file-aware agent
+
+## Files
+
+- `main.py` - Main demo script showing file upload and streaming
+- `example-on-disk.txt` - Sample text file for upload demonstration
+- `memgpt.pdf` - MemGPT paper (downloaded automatically)
+
+## Setup
+
+1. Set your Letta API key: `export LETTA_API_KEY=your_key_here`
+2. Install dependencies: `pip install letta-client requests rich`
+3. Run the demo: `python main.py`
+
+## Usage
+
+The demo will:
+1. Create a data source called "Example Source"
+2. Upload the example text file and PDF
+3. Create an agent named "Clippy"
+4. Start an interactive chat session
+
+Type 'quit' or 'exit' to end the conversation.