feat: add auto-install for missing binaries on register

- New config: auto_install.enabled (default true) and auto_install.silent
- When a plugin registers a binary via glaze.register() and it's missing,
  glaze automatically installs it in the background
- Can be disabled with auto_install = { enabled = false }
- Updated README and help docs

README.md

@@ -128,6 +128,10 @@ require("glaze").setup({
   },
   concurrency = 4,  -- Max parallel installations
   go_cmd = { "go" },  -- Auto-detects goenv if available
+  auto_install = {
+    enabled = true,      -- Auto-install missing binaries on register
+    silent = false,      -- Suppress install notifications
+  },
   auto_check = {
     enabled = true,      -- Auto-check for updates
     frequency = "daily", -- "daily", "weekly", or hours as number