RFC 0001-P4: Agent installers (Linux/macOS + Windows)

Phase 4: end-user installer flows for the on-prem `fleet-agent` binary on Linux/macOS and Windows.

## Acceptance criteria

- `deployment-files/agent-install.sh` for Linux/macOS: downloads or extracts the agent + plugins, creates `/etc/fleet-agent/`, prompts for `--server-url` and runs the challenge-response enrollment, drops a `systemd` unit (or `launchd` plist on macOS).
- `deployment-files/agent-install.ps1` for Windows: same flow plus Windows service registration.
- Both installers handle agent restart/upgrade without losing the host keys (preserves `~/.fleet-agent/identity.key`, `miner_signing.key`, `org_data.key`).
- Companion `agent-uninstall.sh` / `.ps1`: stop service, remove binaries; key files left in place by default with a `--purge` flag to remove them.
- Smoke test: install on a fresh VM, complete enrollment via the UI, agent appears connected.

## Reference

RFC 0001 Phase 4: https://github.com/block/proto-fleet/pull/145