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.

hermes_cli/config.py

@@ -122,6 +122,11 @@ DEFAULT_CONFIG = {
         "user_char_limit": 1375,     # ~500 tokens at 2.75 chars/token
     },
     
+    # Ephemeral prefill messages file — JSON list of {role, content} dicts
+    # injected at the start of every API call for few-shot priming.
+    # Never saved to sessions, logs, or trajectories.
+    "prefill_messages_file": "",
+    
     # Permanently allowed dangerous command patterns (added via "always" approval)
     "command_allowlist": [],
     
@@ -312,6 +317,20 @@ OPTIONAL_ENV_VARS = {
         "password": False,
         "category": "setting",
     },
+    "HERMES_PREFILL_MESSAGES_FILE": {
+        "description": "Path to JSON file with ephemeral prefill messages for few-shot priming",
+        "prompt": "Prefill messages file path",
+        "url": None,
+        "password": False,
+        "category": "setting",
+    },
+    "HERMES_EPHEMERAL_SYSTEM_PROMPT": {
+        "description": "Ephemeral system prompt injected at API-call time (never persisted to sessions)",
+        "prompt": "Ephemeral system prompt",
+        "url": None,
+        "password": False,
+        "category": "setting",
+    },
 }