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/d.xml

@@ -15,10 +15,10 @@
       <rule pattern="//.*?\n">
         <token type="CommentSingle"/>
       </rule>
-      <rule pattern="/\*.*?\*/">
+      <rule pattern="/\*[\s\S]*?\*/">
         <token type="CommentMultiline"/>
       </rule>
-      <rule pattern="/\+.*?\+/">
+      <rule pattern="/\+[\s\S]*?\+/">
         <token type="CommentMultiline"/>
       </rule>
       <rule pattern="(asm|assert|body|break|case|cast|catch|continue|default|debug|delete|do|else|finally|for|foreach|foreach_reverse|goto|if|in|invariant|is|macro|mixin|new|out|pragma|return|super|switch|this|throw|try|typeid|typeof|version|while|with)\b">
@@ -130,4 +130,4 @@
       </rule>
     </state>
   </rules>
-</lexer>
+</lexer>