Add scripts/lmcp-install-macos.sh + LMCP_TOKEN env fallback

lmcp.lua: if opts.auth_token and opts.conf are both unset, fall back to
the LMCP_TOKEN environment variable. Empty string treated as unset.
This is the primitive launchd/systemd drop-ins need — no conf file
bookkeeping on hosts that don't already use one.

scripts/lmcp-install-macos.sh: macOS installer via Homebrew. Drops the
Lua library files into $(brew --prefix)/share/lua/5.4/, mints (or
reuses) a Bearer token stored at $(brew --prefix)/etc/lmcp/token,
installs a ~/Library/LaunchAgents/ plist with LMCP_TOKEN baked in,
launchctl-loads it, and smoke-tests. Prints the Claude Code ~/.claude.json
snippet at the end.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lmcp.lua

@@ -33,12 +33,17 @@ function lmcp.new(name, opts)
     self.port = opts.port or 8080
     self.tools = {}
     self._session_id = nil
-    -- Auth: explicit opt > conf file > nil (no auth)
+    -- Auth: explicit opt > conf file > LMCP_TOKEN env > nil (no auth)
     if opts.auth_token then
         self._auth_token = opts.auth_token
     elseif opts.conf then
         local conf = read_conf(opts.conf)
         self._auth_token = conf['.godparticle']
+    else
+        local env_token = os.getenv("LMCP_TOKEN")
+        if env_token and env_token ~= "" then
+            self._auth_token = env_token
+        end
     end
     return self
 end