refactor(conform): format on demand, not on save

Agents run stylua/biome/rustfmt themselves, and a save-time reformat
races their in-flight writes to the same buffer. Timeout and LSP
fallback move to default_format_opts so <leader>fm (NvChad's binding)
keeps the same behaviour; conform now lazy-loads on first use.

Drops the /contrib/ skip — explicit invocation is already the opt-in.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

lua/plugins/init.lua

@@ -1,7 +1,7 @@
 return {
   {
     "stevearc/conform.nvim",
-    event = "BufWritePre",
+    cmd = "ConformInfo",
     opts = require "configs.conform",
   },
 }