feat(integration): add universal merkwerk binary detection

- Check development binary (./bin/merkwerk)
- Check installed binary (/usr/local/bin/merkwerk)
- Fallback to PATH lookup (command -v merkwerk)
- Proper error handling for missing binary

Related to DAW/furt#94

integrations/lua-api.lua

@@ -10,10 +10,51 @@ local cache = {
     ttl = 300  -- 5 minutes default TTL
 }
 
+-- Find merkwerk binary using universal detection pattern
+local function find_merkwerk_binary()
+    -- Check development binary
+    local dev_handle = io.popen("test -x './bin/merkwerk' && echo './bin/merkwerk' 2>/dev/null")
+    if dev_handle then
+        local dev_result = dev_handle:read("*line")
+        dev_handle:close()
+        if dev_result and dev_result ~= "" then
+            return dev_result
+        end
+    end
+
+    -- Check installed binary
+    local inst_handle = io.popen("test -x '/usr/local/bin/merkwerk' && echo '/usr/local/bin/merkwerk' 2>/dev/null")
+    if inst_handle then
+        local inst_result = inst_handle:read("*line")
+        inst_handle:close()
+        if inst_result and inst_result ~= "" then
+            return inst_result
+        end
+    end
+
+    -- Check PATH
+    local path_handle = io.popen("command -v merkwerk 2>/dev/null")
+    if path_handle then
+        local path_result = path_handle:read("*line")
+        path_handle:close()
+        if path_result and path_result ~= "" then
+            return "merkwerk"
+        end
+    end
+
+    return nil
+end
+
 -- Execute merkwerk command and return result
 local function execute_merkwerk(args)
     args = args or "info --json"
-    local command = "./tools/merkwerk/bin/merkwerk " .. args .. " 2>/dev/null"
+
+    local merkwerk_cmd = find_merkwerk_binary()
+    if not merkwerk_cmd then
+        return nil, "merkwerk binary not found"
+    end
+
+    local command = merkwerk_cmd .. " " .. args .. " 2>/dev/null"
 
     local handle = io.popen(command)
     if not handle then