Add a lines-of-code badge

tokei.rs with `category=code`, so the figure is code lines rather than the
total including blanks and comments — the number people mean by "LOC", and the
one that does not inflate itself when the file is reformatted.

Measured locally at the time of writing, for whoever wants to sanity-check the
badge against the tree: 117 files and 22,585 code lines under `grapharc/`, plus
52 files and 21,016 under `tests/` — 43,601 in total across 169 files, from
56,311 raw lines. Those figures are a snapshot and deliberately not quoted in
the README prose; the badge re-derives its own.

Not verified rendering: tokei.rs and img.shields.io are both unreachable from
the environment this was written in, so the URL is the documented form rather
than one observed to return an image.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: claude

README.md

@@ -12,6 +12,7 @@
 [![Python](https://img.shields.io/pypi/pyversions/grapharc.svg)](https://pypi.org/project/grapharc/)
 [![CI](https://github.com/CodeGraphContext/GraphARC/actions/workflows/ci.yml/badge.svg)](https://github.com/CodeGraphContext/GraphARC/actions/workflows/ci.yml)
 [![License](https://img.shields.io/badge/license-MIT-blue.svg)](LICENSE)
+[![Lines of code](https://tokei.rs/b1/github/CodeGraphContext/GraphARC?category=code)](https://github.com/CodeGraphContext/GraphARC)
 
 **A governed agent runtime built on [LangGraph](https://github.com/langchain-ai/langgraph).**