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/bash.xml

@@ -12,6 +12,7 @@
     <filename>*.ebuild</filename>
     <filename>*.eclass</filename>
     <filename>.env</filename>
+    <filename>.env.*</filename>
     <filename>*.env</filename>
     <filename>*.exheres-0</filename>
     <filename>*.exlib</filename>
@@ -23,6 +24,7 @@
     <filename>bash_*</filename>
     <filename>zshrc</filename>
     <filename>.zshrc</filename>
+    <filename>APKBUILD</filename>
     <filename>PKGBUILD</filename>
     <mime_type>application/x-sh</mime_type>
     <mime_type>application/x-shellscript</mime_type>
@@ -217,4 +219,4 @@
       </rule>
     </state>
   </rules>
-</lexer>
+</lexer>