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/Makefile

@@ -1,21 +1,21 @@
 BUMP_VERSION := $(GOPATH)/bin/bump_version
-STATICCHECK := $(GOPATH)/bin/staticcheck
 WRITE_MAILMAP := $(GOPATH)/bin/write_mailmap
 
-$(STATICCHECK):
-	go get honnef.co/go/tools/cmd/staticcheck
-
-lint: $(STATICCHECK)
+lint:
 	go vet ./...
-	$(STATICCHECK)
+	go run honnef.co/go/tools/cmd/staticcheck@latest ./...
 
-test: lint
+test:
 	@# the timeout helps guard against infinite recursion
 	go test -timeout=250ms ./...
 
-race-test: lint
+race-test:
 	go test -timeout=500ms -race ./...
 
+coverage:
+	go test -trimpath -timeout=250ms -coverprofile=coverage.out -covermode=atomic ./...
+	go tool cover -func=coverage.out
+
 $(BUMP_VERSION):
 	go get -u github.com/kevinburke/bump_version