fix(service): enhance message filtering to drop empty chat messages while retaining tool requests

QUICKSTART.md

@@ -285,10 +285,10 @@ LLM → metrics) before exiting.
   call them from your agent loop.
 - **App + project scope** — set `app_id` / `project_id` to anything
   other than `"default"` to partition memory spaces inside one server.
-- **Multi-modal messages** — `messages[].content` accepts a list of
-  typed `ContentItem`s (`text` / `image` / `audio` / `doc` / `pdf` /
-  `html` / `email`) for non-text input. Install the optional extra
-  to enable parsing:
+- **Mixed content messages** — `messages[].content` accepts a list of
+  typed `ContentItem`s (`text` / `md` / `image` / `audio` / `doc` /
+  `pdf` / `html` / `email`). Markdown (`md`) is read as UTF-8 text.
+  Install the optional extra to enable parsing for media/doc types:
   `uv pip install 'everos[multimodal]'`. Office documents
   (`doc` / `docx` / `xls` / `ppt` / `…`) additionally need
   **LibreOffice** on the host (`brew install --cask libreoffice` /