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/golang.org/x/sys/windows/env_windows.go

@@ -37,14 +37,17 @@ func (token Token) Environ(inheritExisting bool) (env []string, err error) {
 		return nil, err
 	}
 	defer DestroyEnvironmentBlock(block)
-	blockp := unsafe.Pointer(block)
-	for {
-		entry := UTF16PtrToString((*uint16)(blockp))
-		if len(entry) == 0 {
-			break
+	size := unsafe.Sizeof(*block)
+	for *block != 0 {
+		// find NUL terminator
+		end := unsafe.Pointer(block)
+		for *(*uint16)(end) != 0 {
+			end = unsafe.Add(end, size)
 		}
-		env = append(env, entry)
-		blockp = unsafe.Add(blockp, 2*(len(entry)+1))
+
+		entry := unsafe.Slice(block, (uintptr(end)-uintptr(unsafe.Pointer(block)))/size)
+		env = append(env, UTF16ToString(entry))
+		block = (*uint16)(unsafe.Add(end, size))
 	}
 	return env, nil
 }