deploy(hf): inject HF Spaces frontmatter into README for hf-deploy branch

hf/main is 20+ commits stale (pre-v1 state). Force-pushing v1 to hf/main
would overwrite the HF Space frontmatter (sdk: docker, app_port: 7860)
and break the build. This commit lives only on the hf-deploy branch,
which is the only branch pushed to hf/main. origin/main's README stays
clean of HF-specific frontmatter.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -1,3 +1,12 @@
+---
+title: agent-bench
+emoji: "🔍"
+colorFrom: blue
+colorTo: purple
+sdk: docker
+app_port: 7860
+---
+
 # agent-bench
 
 **A RAG benchmark built from primitives, with honest evaluation of retrieval, refusal, and grounded citation.**