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/.goreleaser.yml

@@ -4,34 +4,31 @@ release:
     owner: alecthomas
     name: chroma
 brews:
-  -
-    install: bin.install "chroma"
+  - install: bin.install "chroma"
 env:
   - CGO_ENABLED=0
 builds:
-- goos:
-    - linux
-    - darwin
-    - windows
-  goarch:
-    - arm64
-    - amd64
-    - "386"
-  goarm:
-    - "6"
-  dir: ./cmd/chroma
-  main: .
-  ldflags: -s -w -X main.version={{.Version}} -X main.commit={{.Commit}} -X main.date={{.Date}}
-  binary: chroma
+  - goos:
+      - linux
+      - darwin
+      - windows
+    goarch:
+      - arm64
+      - amd64
+      - "386"
+    goarm:
+      - "6"
+    dir: ./cmd/chroma
+    main: .
+    ldflags: -s -w -X main.version={{.Version}} -X main.commit={{.Commit}} -X main.date={{.Date}}
+    binary: chroma
 archives:
-  -
-    format: tar.gz
-    name_template: '{{ .Binary }}-{{ .Version }}-{{ .Os }}-{{ .Arch }}{{ if .Arm }}v{{
-    .Arm }}{{ end }}'
+  - format: tar.gz
+    name_template: "{{ .Binary }}-{{ .Version }}-{{ .Os }}-{{ .Arch }}{{ if .Arm }}v{{ .Arm }}{{ end }}"
     files:
       - COPYING
       - README*
 snapshot:
   name_template: SNAPSHOT-{{ .Commit }}
 checksum:
-  name_template: '{{ .ProjectName }}-{{ .Version }}-checksums.txt'
+  name_template: "{{ .ProjectName }}-{{ .Version }}-checksums.txt"