docs: add subtitle under README title for at-a-glance identity

"HalfSeed" alone is a codename that doesn't convey the project's
purpose. The new subtitle ("Multi-agent LLM research orchestrator ·
local-first · self-hosted · emits a real paper.tex") gives scanners
the three-word identity in the first half-second of looking at the
page, before badges and tagline.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: HaibaraAi137

README.md

@@ -1,5 +1,7 @@
 # HalfSeed
 
+**Multi-agent LLM research orchestrator** · *local-first · self-hosted · emits a real `paper.tex`*
+
 [![backend](https://github.com/HaibaraAi137/HalfSeed/actions/workflows/backend.yml/badge.svg)](https://github.com/HaibaraAi137/HalfSeed/actions/workflows/backend.yml)
 [![frontend](https://github.com/HaibaraAi137/HalfSeed/actions/workflows/frontend.yml/badge.svg)](https://github.com/HaibaraAi137/HalfSeed/actions/workflows/frontend.yml)
 [![docker](https://github.com/HaibaraAi137/HalfSeed/actions/workflows/docker.yml/badge.svg)](https://github.com/HaibaraAi137/HalfSeed/actions/workflows/docker.yml)