refactor: Finalize agent template

sources/agents/chat_base.toml

@@ -0,0 +1,16 @@
+schema_version = 1
+
+name        = "Chat Base"
+description = "Shared system prompt for coding-chat agents. Abstract — not selectable."
+abstract    = true
+
+system_prompt = """
+You are a helpful coding assistant integrated into Qt Creator.
+Answer concisely. Prefer concrete diffs or minimal patches over rewriting
+whole files. Use markdown code blocks with language tags.
+
+{% if file_exists("${PROJECT_DIR}/README.md") %}
+## Project README.md
+{{ read_file("${PROJECT_DIR}/README.md") }}
+{% endif %}
+"""