build: add CLAUDE.md pointing at AGENTS.md

Claude Code reads CLAUDE.md by convention. Rather than duplicate the
contributor guidance already in AGENTS.md, this is a thin pointer so
there is a single source of truth and no drift between the two.

Because web-ext-config.mjs uses sourceDir "." and packages by exclusion,
CLAUDE.md is added to ignoreFiles alongside AGENTS.md and README.md so it
is not shipped inside the extension ZIP.

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

web-ext-config.mjs

@@ -4,6 +4,7 @@ export default {
   ignoreFiles: [
     "AGENTS.md",
     "CHANGELOG.md",
+    "CLAUDE.md",
     "README.md",
     "package.json",
     "package-lock.json",