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/styles/rrt.xml

@@ -10,4 +10,10 @@
   <entry type="LiteralNumber" style="#ff6600"/>
   <entry type="Comment" style="#00ff00"/>
   <entry type="CommentPreproc" style="#e5e5e5"/>
+  <entry type="GenericDeleted" style="#f00"/>
+  <entry type="GenericEmph" style="italic"/>
+  <entry type="GenericHeading" style="bold #ff0"/>
+  <entry type="GenericInserted" style="#0f0"/>
+  <entry type="GenericStrong" style="bold"/>
+  <entry type="GenericSubheading" style="bold #87ceeb"/>
 </style>