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/python.xml

@@ -6,6 +6,7 @@
     <alias>sage</alias>
     <alias>python3</alias>
     <alias>py3</alias>
+    <alias>starlark</alias>
     <filename>*.py</filename>
     <filename>*.pyi</filename>
     <filename>*.pyw</filename>
@@ -19,6 +20,11 @@
     <filename>BUILD</filename>
     <filename>BUILD.bazel</filename>
     <filename>WORKSPACE</filename>
+    <filename>WORKSPACE.bzlmod</filename>
+    <filename>WORKSPACE.bazel</filename>
+    <filename>MODULE.bazel</filename>
+    <filename>REPO.bazel</filename>
+    <filename>*.star</filename>
     <filename>*.tac</filename>
     <mime_type>text/x-python</mime_type>
     <mime_type>application/x-python</mime_type>
@@ -586,4 +592,4 @@
       </rule>
     </state>
   </rules>
-</lexer>
+</lexer>