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/dario.cat/mergo/SECURITY.md

@@ -4,8 +4,8 @@
 
 | Version | Supported          |
 | ------- | ------------------ |
-| 0.3.x   | :white_check_mark: |
-| < 0.3   | :x:                |
+| 1.x.x   | :white_check_mark: |
+| < 1.0   | :x:                |
 
 ## Security contact information