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/yaml.xml

@@ -53,7 +53,7 @@
         <bygroups>
           <token type="Punctuation"/>
           <token type="LiteralStringDoc"/>
-          <token type="TextWhitespace"/>
+          <token type="Ignore"/>
         </bygroups>
       </rule>
       <rule pattern="(false|False|FALSE|true|True|TRUE|null|Off|off|yes|Yes|YES|OFF|On|ON|no|No|on|NO|n|N|Y|y)\b">