chore: modernize CI and update Go toolchain

- Bump Go from 1.19 to 1.26 and update all dependencies
- Rewrite CI workflow with matrix strategy (Linux, macOS, Windows)
- Update GitHub Actions to current versions (checkout@v4, setup-go@v5)
- Update CodeQL actions from v1 to v3
- Fix cross-platform bug in mock/path.go (path.Join -> filepath.Join)
- Clean up dependabot config (weekly schedule, remove stale ignore)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vendor/github.com/alecthomas/chroma/v2/delegate.go

@@ -24,6 +24,15 @@ func DelegatingLexer(root Lexer, language Lexer) Lexer {
 	}
 }
 
+func (d *delegatingLexer) SetTracing(enable bool) {
+	if l, ok := d.language.(TracingLexer); ok {
+		l.SetTracing(enable)
+	}
+	if l, ok := d.root.(TracingLexer); ok {
+		l.SetTracing(enable)
+	}
+}
+
 func (d *delegatingLexer) AnalyseText(text string) float32 {
 	return d.root.AnalyseText(text)
 }