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/lexers/gemtext.go

@@ -0,0 +1,37 @@
+package lexers
+
+import (
+	. "github.com/alecthomas/chroma/v2" // nolint
+)
+
+// Gemtext lexer.
+var Gemtext = Register(MustNewLexer(
+	&Config{
+		Name:      "Gemtext",
+		Aliases:   []string{"gemtext", "gmi", "gmni", "gemini"},
+		Filenames: []string{"*.gmi", "*.gmni", "*.gemini"},
+		MimeTypes: []string{"text/gemini"},
+	},
+	gemtextRules,
+))
+
+func gemtextRules() Rules {
+	return Rules{
+		"root": {
+			{`^(#[^#].+\r?\n)`, ByGroups(GenericHeading), nil},
+			{`^(#{2,3}.+\r?\n)`, ByGroups(GenericSubheading), nil},
+			{`^(\* )(.+\r?\n)`, ByGroups(Keyword, Text), nil},
+			{`^(>)(.+\r?\n)`, ByGroups(Keyword, GenericEmph), nil},
+			{"^(```\\r?\\n)([\\w\\W]*?)(^```)(.+\\r?\\n)?", ByGroups(String, Text, String, Comment), nil},
+			{
+				"^(```)(\\w+)(\\r?\\n)([\\w\\W]*?)(^```)(.+\\r?\\n)?",
+				UsingByGroup(2, 4, String, String, String, Text, String, Comment),
+				nil,
+			},
+			{"^(```)(.+\\r?\\n)([\\w\\W]*?)(^```)(.+\\r?\\n)?", ByGroups(String, String, Text, String, Comment), nil},
+			{`^(=>)(\s*)([^\s]+)(\s*)$`, ByGroups(Keyword, Text, NameAttribute, Text), nil},
+			{`^(=>)(\s*)([^\s]+)(\s+)(.+)$`, ByGroups(Keyword, Text, NameAttribute, Text, NameTag), nil},
+			{`(.|(?:\r?\n))`, ByGroups(Text), nil},
+		},
+	}
+}