chore: drop redundant CLAUDE.md, keep AGENTS.md only

CLAUDE.md contained nothing but the generated <!-- gitnexus --> block,
which is already present verbatim in AGENTS.md. Tools that read both
prefer CLAUDE.md, so the stub was shadowing this repo's actual
conventions.

Remove it and gitignore the root path so a future `gitnexus analyze`
cannot reintroduce it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: Patel230

.gitignore

@@ -53,3 +53,8 @@ rho-sec105b.log
 
 # Compiled Go test binaries
 *.test
+
+# Agent instructions: this repo is AGENTS.md-only.
+# `gitnexus analyze` regenerates a root CLAUDE.md that would shadow AGENTS.md
+# for tools that prefer it; keep the regenerated copy out of git.
+/CLAUDE.md