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/go-git/go-billy/v5/Makefile

@@ -1,6 +1,7 @@
 # Go parameters
 GOCMD = go
 GOTEST = $(GOCMD) test 
+WASIRUN_WRAPPER := $(CURDIR)/scripts/wasirun-wrapper
 
 .PHONY: test
 test:
@@ -9,3 +10,9 @@ test:
 test-coverage:
 	echo "" > $(COVERAGE_REPORT); \
 	$(GOTEST) -coverprofile=$(COVERAGE_REPORT) -coverpkg=./... -covermode=$(COVERAGE_MODE) ./...
+
+.PHONY: wasitest
+wasitest: export GOARCH=wasm
+wasitest: export GOOS=wasip1
+wasitest:
+	$(GOTEST) -exec $(WASIRUN_WRAPPER) ./...