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/cloudflare/circl/math/fp448/fp_amd64.s

@@ -1,4 +1,5 @@
-// +build amd64
+//go:build amd64 && !purego
+// +build amd64,!purego
 
 #include "textflag.h"
 #include "fp_amd64.h"