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/kevinburke/ssh_config/AUTHORS.txt

@@ -5,5 +5,7 @@ Kevin Burke <kevin@burke.dev>
 Mark Nevill <nev@improbable.io>
 Scott Lessans <slessans@gmail.com>
 Sergey Lukjanov <me@slukjanov.name>
+Simon Josefsson <simon@josefsson.org>
+sio2boss <sio2boss@users.noreply.github.com>
 Wayne Ashley Berry <wayneashleyberry@gmail.com>
 santosh653 <70637961+santosh653@users.noreply.github.com>