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.

tests/watchexec/watchexec_spec.lua

@@ -1,17 +1,31 @@
-local plugin = require("watchexec")
-
-describe("watchexec logic", function()
-  before_each(function()
-    plugin.setup({
-      greeting = "Hello Test!",
-    })
-  end)
+local watchexec = require("watchexec")
 
+describe("watchexec module", function()
   it("can be required without errors", function()
-    assert.not_nil(plugin)
+    assert.not_nil(watchexec)
   end)
 
-  it("correctly applies user configuration", function()
-    assert.equals("Hello Test!", plugin.config.greeting)
+  it("exposes setup function", function()
+    assert.is_function(watchexec.setup)
+  end)
+
+  it("exposes run function", function()
+    assert.is_function(watchexec.run)
+  end)
+
+  it("exposes stop function", function()
+    assert.is_function(watchexec.stop)
+  end)
+
+  it("exposes toggle function", function()
+    assert.is_function(watchexec.toggle)
+  end)
+
+  it("setup delegates to config", function()
+    local config = require("watchexec.config")
+
+    watchexec.setup({ auto_scroll = false })
+
+    assert.equals(false, config.get().auto_scroll)
   end)
 end)