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/lua.xml

@@ -2,8 +2,10 @@
   <config>
     <name>Lua</name>
     <alias>lua</alias>
+    <alias>luau</alias>
     <filename>*.lua</filename>
     <filename>*.wlua</filename>
+    <filename>*.luau</filename>
     <mime_type>text/x-lua</mime_type>
     <mime_type>application/x-lua</mime_type>
   </config>