Implement self-update capability for the unified pulse-agent binary:
- Add internal/agentupdate package with cross-platform update logic
- Hourly version checks against /api/agent/version endpoint
- SHA256 checksum verification for downloaded binaries
- Atomic binary replacement with backup/rollback on failure
- Support for Linux, macOS, and Windows (10 platform/arch combinations)
Build and release changes:
- Dockerfile builds unified agent for all platforms
- build-release.sh includes unified agent in release artifacts
- validate-release.sh validates unified agent binaries
- Install scripts (install.sh, install.ps1) use correct URL format
Related to #727, #737
Fixes#755. Adds interactive pauses and graphical popups (where available) to installer scripts when critical errors occur, ensuring troubleshooting guides are readable. Also clarifies 'build from source' instructions.
