feat: implement public API and plugin commands

Rewrite init.lua to expose setup(), run(), stop(), and toggle() that
coordinate across config, runner, window, and indicator modules. Replace
the placeholder SayHello command in plugin/watchexec.lua with keymaps
(<Leader>wx{t,s,r}), :WatchexecRun/:WatchexecStop/:WatchexecToggle
commands, VimLeavePre cleanup, and VimResized auto-resize.

plugin/watchexec.lua

@@ -1,9 +1,64 @@
+---@type boolean|nil
 if vim.g.loaded_watchexec then
   return
 end
 
 vim.g.loaded_watchexec = 1
 
-vim.api.nvim_create_user_command("SayHello", function()
-  require("watchexec").say_hello()
+vim.keymap.set("n", "<Leader>wxt", function()
+  require("watchexec").toggle()
+end, { desc = "Toggle watchexec window" })
+
+vim.keymap.set("n", "<Leader>wxs", function()
+  require("watchexec").stop()
+end, { desc = "Stop watchexec" })
+
+vim.keymap.set("n", "<Leader>wxr", function()
+  local cmd = vim.fn.input("Watchexec: ")
+
+  if cmd and #cmd > 0 then
+    require("watchexec").run(cmd)
+  end
+end, { desc = "Run watchexec with prompt" })
+
+vim.api.nvim_create_user_command("WatchexecRun", function(opts)
+  require("watchexec").run(opts.args)
+end, { nargs = 1, complete = "file" })
+
+vim.api.nvim_create_user_command("WatchexecStop", function()
+  require("watchexec").stop()
 end, {})
+
+vim.api.nvim_create_user_command("WatchexecToggle", function()
+  require("watchexec").toggle()
+end, {})
+
+vim.api.nvim_set_hl(0, "WatchexecSuccess", { bg = "#00ff00", default = true })
+vim.api.nvim_set_hl(0, "WatchexecFailure", { bg = "#ff0000", default = true })
+
+vim.api.nvim_create_augroup("watchexec_nvim", { clear = true })
+
+vim.api.nvim_create_autocmd("VimLeavePre", {
+  group = "watchexec_nvim",
+  callback = function()
+    require("watchexec.runner").stop()
+  end,
+})
+
+---@type table|nil
+local resize_timer
+
+vim.api.nvim_create_autocmd("VimResized", {
+  group = "watchexec_nvim",
+  callback = function()
+    if resize_timer then
+      resize_timer:close()
+    end
+
+    resize_timer = vim.defer_fn(function()
+      resize_timer = nil
+      require("watchexec.window").resize_float()
+      require("watchexec.indicator").reposition()
+    end, 100)
+  end,
+})