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-latte.xml

@@ -5,7 +5,7 @@
   <entry type="Other" style="#4c4f69"/>
   <entry type="LineTableTD" style=""/>
   <entry type="LineTable" style=""/>
-  <entry type="LineHighlight" style="#bcc0cc"/>
+  <entry type="LineHighlight" style="bg:#bcc0cc"/>
   <entry type="LineNumbersTable" style="#8c8fa1"/>
   <entry type="LineNumbers" style="#8c8fa1"/>
   <entry type="Keyword" style="#8839ef"/>
@@ -65,12 +65,12 @@
   <entry type="CommentSingle" style="italic #9ca0b0"/>
   <entry type="CommentMultiline" style="italic #9ca0b0"/>
   <entry type="CommentSpecial" style="italic #9ca0b0"/>
-  <entry type="CommentHashbang" style="italic #9ca0b0"/>
+  <entry type="CommentHashbang" style="italic #acb0be"/>
   <entry type="CommentPreproc" style="italic #9ca0b0"/>
   <entry type="CommentPreprocFile" style="bold #9ca0b0"/>
   <entry type="Generic" style="#4c4f69"/>
   <entry type="GenericInserted" style="bg:#ccd0da #40a02b"/>
-  <entry type="GenericDeleted" style="#d20f39 bg:#ccd0da"/>
+  <entry type="GenericDeleted" style="bg:#ccd0da #d20f39"/>
   <entry type="GenericEmph" style="italic #4c4f69"/>
   <entry type="GenericStrong" style="bold #4c4f69"/>
   <entry type="GenericUnderline" style="underline #4c4f69"/>
@@ -80,4 +80,4 @@
   <entry type="GenericPrompt" style="#4c4f69"/>
   <entry type="GenericError" style="#d20f39"/>
   <entry type="GenericTraceback" style="#d20f39"/>
-</style>
+</style>