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/pjbgf/sha1cd/Dockerfile.arm

@@ -1,4 +1,4 @@
-FROM golang:1.20@sha256:2edf6aab2d57644f3fe7407132a0d1770846867465a39c2083770cf62734b05d
+FROM golang:1.24@sha256:14fd8a55e59a560704e5fc44970b301d00d344e45d6b914dda228e09f359a088
 
 ENV GOOS=linux
 ENV GOARCH=arm
@@ -10,7 +10,6 @@ ENV PKG_CONFIG_PATH=/usr/lib/arm-linux-gnueabihf/pkgconfig
 RUN dpkg --add-architecture armhf \
     && apt update \
     && apt install -y --no-install-recommends \
-        upx \
         gcc-arm-linux-gnueabihf \
         libc6-dev-armhf-cross \
         pkg-config \