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/README.md

@@ -82,11 +82,11 @@ file format.
 [blog]: https://kev.inburke.com/kevin/more-comment-preserving-configuration-parsers/
 [hostsfile]: https://github.com/kevinburke/hostsfile
 
-## Donating
+## Sponsorships
 
-I don't get paid to maintain this project. Donations free up time to make
-improvements to the library, and respond to bug reports. You can send donations
-via Paypal's "Send Money" feature to kev@inburke.com. Donations are not tax
-deductible in the USA.
+Thank you very much to Tailscale and Indeed for sponsoring development of this
+library. [Sponsors][sponsors] will get their names featured in the README.
 
 You can also reach out about a consulting engagement: https://burke.services
+
+[sponsors]: https://github.com/sponsors/kevinburke