feat: add ephemeral prefill messages and system prompt loading

- Implemented functionality to load ephemeral prefill messages from a JSON file, enhancing few-shot priming capabilities for the agent.
- Introduced a mechanism to load an ephemeral system prompt from environment variables or configuration files, ensuring dynamic prompt adjustments at API-call time.
- Updated the CLI and agent initialization to utilize the new prefill messages and system prompt, improving the overall interaction experience.
- Enhanced configuration options with new environment variables for prefill messages and system prompts, allowing for greater customization without persistence.

tools/file_tools.py

@@ -199,7 +199,7 @@ def _check_file_reqs():
 
 READ_FILE_SCHEMA = {
     "name": "read_file",
-    "description": "Read a file with line numbers and pagination. Use this instead of cat/head/tail in terminal. Output format: 'LINE_NUM|CONTENT'. Suggests similar filenames if not found. Images (png/jpg/gif/webp) returned as base64. Use offset and limit for large files.",
+    "description": "Read a text file with line numbers and pagination. Use this instead of cat/head/tail in terminal. Output format: 'LINE_NUM|CONTENT'. Suggests similar filenames if not found. Use offset and limit for large files. NOTE: Cannot read images or binary files — use vision_analyze for images.",
     "parameters": {
         "type": "object",
         "properties": {