Adds ARM (Raspberry Pi) cross-compilation support for harmony_agent
and harmony_inventory_agent.
- .cargo/config.toml: aarch64-linux-gnu-gcc linker
- build/cross-arm.sh: local cross-build script with prereq checks
- .gitea/workflows/arm-agents.yaml: CI workflow that builds both
agents for aarch64 on tagged releases and uploads them as
release assets (same pattern as harmony_inventory_agent already
uses)
Removes the unused 'harmony' dependency from harmony_agent — it
was vestigial (no actual imports) and was the only thing pulling
in heavy C deps (libvirt via virt) that blocked ARM builds.
