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/dlclark/regexp2/runner.go

@@ -1604,6 +1604,10 @@ func (re *Regexp) getRunner() *runner {
 // run using re.  (The cache empties when re gets garbage collected.)
 func (re *Regexp) putRunner(r *runner) {
 	re.muRun.Lock()
+	r.runtext = nil
+	if r.runmatch != nil {
+		r.runmatch.text = nil
+	}
 	re.runner = append(re.runner, r)
 	re.muRun.Unlock()
 }