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/presets_spec.lua

@@ -13,6 +13,10 @@ describe('presets', function()
   }
 
   describe('typst', function()
+    it('has ft', function()
+      assert.are.equal('typst', presets.typst.ft)
+    end)
+
     it('has cmd', function()
       assert.are.same({ 'typst', 'compile' }, presets.typst.cmd)
     end)
@@ -28,6 +32,10 @@ describe('presets', function()
       assert.is_string(output)
       assert.are.equal('/tmp/document.pdf', output)
     end)
+
+    it('has open enabled', function()
+      assert.are.same({ 'xdg-open' }, presets.typst.open)
+    end)
   end)
 
   describe('latex', function()
@@ -38,6 +46,10 @@ describe('presets', function()
       ft = 'tex',
     }
 
+    it('has ft', function()
+      assert.are.equal('tex', presets.latex.ft)
+    end)
+
     it('has cmd', function()
       assert.are.same({ 'latexmk' }, presets.latex.cmd)
     end)
@@ -59,6 +71,10 @@ describe('presets', function()
       assert.is_table(clean)
       assert.are.same({ 'latexmk', '-c', '/tmp/document.tex' }, clean)
     end)
+
+    it('has open enabled', function()
+      assert.are.same({ 'xdg-open' }, presets.latex.open)
+    end)
   end)
 
   describe('markdown', function()
@@ -69,20 +85,84 @@ describe('presets', function()
       ft = 'markdown',
     }
 
+    it('has ft', function()
+      assert.are.equal('markdown', presets.markdown.ft)
+    end)
+
     it('has cmd', function()
       assert.are.same({ 'pandoc' }, presets.markdown.cmd)
     end)
 
-    it('returns args with file and output flag', function()
+    it('returns args with standalone and embed-resources flags', function()
       local args = presets.markdown.args(md_ctx)
       assert.is_table(args)
-      assert.are.same({ '/tmp/document.md', '-o', '/tmp/document.pdf' }, args)
+      assert.are.same(
+        { '/tmp/document.md', '-s', '--embed-resources', '-o', '/tmp/document.html' },
+        args
+      )
     end)
 
-    it('returns pdf output path', function()
+    it('returns html output path', function()
       local output = presets.markdown.output(md_ctx)
       assert.is_string(output)
-      assert.are.equal('/tmp/document.pdf', output)
+      assert.are.equal('/tmp/document.html', output)
+    end)
+
+    it('returns clean command', function()
+      local clean = presets.markdown.clean(md_ctx)
+      assert.is_table(clean)
+      assert.are.same({ 'rm', '-f', '/tmp/document.html' }, clean)
+    end)
+
+    it('has open enabled', function()
+      assert.are.same({ 'xdg-open' }, presets.markdown.open)
+    end)
+  end)
+
+  describe('github', function()
+    local md_ctx = {
+      bufnr = 1,
+      file = '/tmp/document.md',
+      root = '/tmp',
+      ft = 'markdown',
+    }
+
+    it('has ft', function()
+      assert.are.equal('markdown', presets.github.ft)
+    end)
+
+    it('has cmd', function()
+      assert.are.same({ 'pandoc' }, presets.github.cmd)
+    end)
+
+    it('returns args with standalone, embed-resources, and css flags', function()
+      local args = presets.github.args(md_ctx)
+      assert.is_table(args)
+      assert.are.same({
+        '/tmp/document.md',
+        '-s',
+        '--embed-resources',
+        '--css',
+        'https://cdn.jsdelivr.net/gh/pixelbrackets/gfm-stylesheet@master/github.css',
+        '-o',
+        '/tmp/document.html',
+      }, args)
+    end)
+
+    it('returns html output path', function()
+      local output = presets.github.output(md_ctx)
+      assert.is_string(output)
+      assert.are.equal('/tmp/document.html', output)
+    end)
+
+    it('returns clean command', function()
+      local clean = presets.github.clean(md_ctx)
+      assert.is_table(clean)
+      assert.are.same({ 'rm', '-f', '/tmp/document.html' }, clean)
+    end)
+
+    it('has open enabled', function()
+      assert.are.same({ 'xdg-open' }, presets.github.open)
     end)
   end)
 end)