docs: tidy LLM.md indexes; CLAUDE.md -> LLM.md symlink convention

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CLAUDE.md

@@ -1 +1 @@
-AGENTS.md
+LLM.md

LLM.md

@@ -1 +0,0 @@
-CLAUDE.md

LLM.md

@@ -0,0 +1,8 @@
+# Instructions for agents
+
+- Use `make help` to find available development targets
+- Before committing `.go` changes, run `make fmt` to format, and run `make lint` to lint
+- Before committing `go.mod` changes, run `go mod tidy`
+- Before committing new `.go` files, add the current year into the copyright header
+- Before committing any files, remove all trailing whitespace from source code lines
+- Add test code for newly added functions or new logic of old functions