feat: cross-platform support (Linux, macOS, Windows)

[priyanshujain]

Summary

• Drop CGO: Switch from mattn/go-sqlite3 (CGO) to modernc.org/sqlite (pure Go), fixing the broken Docker build and enabling cross-compilation. Net -1 dependency.
• Cross-platform signals: Build-tagged internal/platform package replaces direct syscall.SIGTERM usage (unavailable on Windows).
• Cross-platform file locking: Split daemon/lock.go into Unix (syscall.Flock) and Windows (kernel32.dll LockFileEx) implementations. Stores the Windows file handle once to avoid os.File.Fd() duplication.
• Windows service manager stub: Returns Task Scheduler instructions for install/uninstall; obk service run works in foreground.
• Cross-platform credentials: macOS keeps Keychain; Linux/Windows use file-based ~/.obk/secrets/ with 0600 permissions.
• Docker multi-arch: Added TARGETOS/TARGETARCH build args, updated CMD to service run.
• CI/CD: GitHub Actions workflow (test on 3 OSes + cross-compile 7 targets), GoReleaser config for automated releases.
• Test fixes: Close WhatsApp clients in tests to release SQLite handles on Windows, check for gcloud accounts before running live tests, handle context cancellation in daemon shutdown test, use USERPROFILE on Windows for home dir in tests.

Verification

All 7 GOOS/GOARCH targets cross-compile:

linux/amd64, linux/arm64, linux/riscv64
darwin/amd64, darwin/arm64
windows/amd64, windows/arm64

CI passes on all 3 OSes (ubuntu, macos, windows). go vet passes on all platforms.

Test plan

• CI passes on ubuntu, macos, windows runners
• Cross-compile job builds all 7 platform targets
• go test ./... passes on all 3 OSes
• Docker build works: docker buildx build --platform linux/amd64,linux/arm64 -t openbotkit:test infrastructure/docker/
• Tag a v* release to verify GoReleaser produces binaries