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/catppuccin-mocha.xml

@@ -5,7 +5,7 @@
   <entry type="Other" style="#cdd6f4"/>
   <entry type="LineTableTD" style=""/>
   <entry type="LineTable" style=""/>
-  <entry type="LineHighlight" style="#45475a"/>
+  <entry type="LineHighlight" style="bg:#45475a"/>
   <entry type="LineNumbersTable" style="#7f849c"/>
   <entry type="LineNumbers" style="#7f849c"/>
   <entry type="Keyword" style="#cba6f7"/>
@@ -65,12 +65,12 @@
   <entry type="CommentSingle" style="italic #6c7086"/>
   <entry type="CommentMultiline" style="italic #6c7086"/>
   <entry type="CommentSpecial" style="italic #6c7086"/>
-  <entry type="CommentHashbang" style="italic #6c7086"/>
+  <entry type="CommentHashbang" style="italic #585b70"/>
   <entry type="CommentPreproc" style="italic #6c7086"/>
   <entry type="CommentPreprocFile" style="bold #6c7086"/>
   <entry type="Generic" style="#cdd6f4"/>
   <entry type="GenericInserted" style="bg:#313244 #a6e3a1"/>
-  <entry type="GenericDeleted" style="#f38ba8 bg:#313244"/>
+  <entry type="GenericDeleted" style="bg:#313244 #f38ba8"/>
   <entry type="GenericEmph" style="italic #cdd6f4"/>
   <entry type="GenericStrong" style="bold #cdd6f4"/>
   <entry type="GenericUnderline" style="underline #cdd6f4"/>
@@ -80,4 +80,4 @@
   <entry type="GenericPrompt" style="#cdd6f4"/>
   <entry type="GenericError" style="#f38ba8"/>
   <entry type="GenericTraceback" style="#f38ba8"/>
-</style>
+</style>