ci: add change-aware builds and Go releases

[samuelbles07]

Summary

Add change-aware CI and tag-driven releases so pull requests build only affected targets while AirGradient Go release tags publish versioned firmware bundles.

Changes

• Add a tested change selector with dynamic firmware and host-test execution
• Skip compilation for documentation-only changes and conservatively build all products for shared or unknown changes
• Pin ESP-IDF v5.5.4, cache managed components, and enforce lockfile stability
• Derive the Go firmware version from Git tags, commits, and working-tree state
• Add a generic release workflow that packages and publishes Go firmware images
• Document CI, versioning, and release behavior

Validation

• Passed 15 build-selector unit tests
• Passed AirGradient Go and Reference firmware builds for the CI change
• Passed the native host-test suite for the CI change
• Verified exact-tag, post-tag, dirty, untagged, and malformed-tag version scenarios
• Passed repository pre-commit checks