feat: rename watch → toggle, auto-compile on start, built-in opener

Problem: :Preview watch only registered a BufWritePost autocmd without
compiling immediately, required boilerplate to open output files after
first compilation, and was misleadingly named.

Solution: Rename watch → toggle throughout. M.toggle now compiles
immediately on activation. Add an open field to ProviderConfig: true
calls vim.ui.open(), a string[] runs the command with the output path
appended, tracked per-buffer so the file opens only once. All presets
default to { 'xdg-open' }. Health check validates opener binaries.
Guard the async compile callback against invalid buffer ids.

spec/commands_spec.lua

@@ -35,10 +35,10 @@ describe('commands', function()
       end)
     end)
 
-    it('does not error on :Preview watch with no provider', function()
+    it('does not error on :Preview toggle with no provider', function()
       require('preview.commands').setup()
       assert.has_no.errors(function()
-        vim.cmd('Preview watch')
+        vim.cmd('Preview toggle')
       end)
     end)
   end)