feat: add transform_llm_output plugin hook

Enables plugins to transform LLM output text after generation,
useful for vocabulary/personality transformation without burning
inference tokens.

Follows same pattern as transform_tool_result and transform_terminal_output:
- First non-empty string result wins
- Fail-open: exceptions logged as warnings, agent continues
- Signature: (response_text, session_id, model, platform)

hermes_cli/plugins.py

@@ -80,6 +80,10 @@ VALID_HOOKS: Set[str] = {
     "post_tool_call",
     "transform_terminal_output",
     "transform_tool_result",
+    # Transform LLM output before it's returned to the user.
+    # Plugins return a string to replace the response text, or None/empty to leave unchanged.
+    # First non-None string wins. Useful for vocabulary/personality transformation.
+    "transform_llm_output",
     "pre_llm_call",
     "post_llm_call",
     "pre_api_request",