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

doc/glaze.txt

@@ -151,6 +151,10 @@ Default configuration:
     },
     concurrency = 4,
     go_cmd = { "go" },  -- Auto-detects goenv
+    auto_install = {
+      enabled = true,      -- Auto-install missing binaries on register
+      silent = false,      -- Suppress install notifications
+    },
     auto_check = {
       enabled = true,      -- Auto-check for updates on setup
       frequency = "daily", -- "daily", "weekly", or hours (number)