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/json.xml

@@ -3,6 +3,8 @@
     <name>JSON</name>
     <alias>json</alias>
     <filename>*.json</filename>
+    <filename>*.jsonc</filename>
+    <filename>*.avsc</filename>
     <mime_type>application/json</mime_type>
     <dot_all>true</dot_all>
     <not_multiline>true</not_multiline>
@@ -107,4 +109,4 @@
       </rule>
     </state>
   </rules>
-</lexer>
+</lexer>