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/embedded/cue.xml

@@ -49,7 +49,7 @@
       <rule pattern="(true|false|null|_)\b">
         <token type="KeywordConstant"/>
       </rule>
-      <rule pattern="[_a-zA-Z]\w*">
+      <rule pattern="[@#]?[_a-zA-Z$]\w*">
         <token type="Name"/>
       </rule>
     </state>